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Meiosis I: Definition, Stages, Phases, and Diagram
Continue ReadingMeiosis I: Introduction
Sexual reproduction is a stepwise evolution of eukaryotes; varying from species to species. The basic mechanism of zygote fusion and meiosis regulators are conserved in all eukaryotes; the difference is seen in the method the gametes meet and evolved along the groups.
Pollination in flowering plants is the means of gametic fusion and produces seeds; which germinates on suitable condition to produce plants.
Plants are exposed to varied errors in cell division and few species under adverse conditions; produces spores which on exposure to suitable conditions proliferates.
Eggs and pollens of plants naturally depends on external mechanical factors for the initial fusion of gametes.
The development and maturation of the plants takes place in a sequential manner on appropriate time and nutrient availabilities.
The basic principles of meiosis include the law of independent assortment with which the plant acquires genetic diversity and variability by crossing over forming a recombinant.
Crossing Over and recombination followed by continuous double cellular division give rise to haploid daughter cells; changes to gametes of male and female in respective parts of a flower of same plant or a different plant.
The reductive division from which the name meiosis was given; in Greek meiosis is to diminish, recombination by crossing over where homologous chromosomes come together to interchange genetic material, and independent assortment of genes to the progeny are the main features a meiotic cell provides to make the process more unique and provide genetic diversity.
Meiosis History
Meiosis was first described by Oscar Hertwig, German Biologist in 1876 further research took place in the field where Theodor Boveri, 1888 reported it in Roundworm.
The reason behind such division was later pointed by August Weismann as to maintain equal amount of genetic material transfer from parent to progeny because if the reproductive division follows mitosis a diploid cell becomes tetraploid and increase exponentially over generation and have resulted in the formation of species with infinite genetic material.
Thomas Hunt Morgan in Drosophila found the recombination of the chromosomes (genetic material) to provide evolutionary variability to organism.
Mitosis vs Meiosis
Both are type of cell division taking place in every eukaryote. But the main difference is, Mitosis produces Diploid Somatic cells identical to parent and Meiosis give rise to cells which forms a new progeny which are genetically different from the parent by producing haploid gametes.
In general; all cells are mitotic and undergoes mitotic cell division; even the gametes before meiosis are divided mitotically from their progenitor and enter meiosis.
“The Molecular Switch” present in every cell when induced on appropriate nutrients makes the cell competent to enter meiotic cycle; is the mechanism to “turn on” meiosis at the beginning of Phase G2.
From G2 phase the meiosis proceeds by 2 continuous cell division.
Further; the phases of meiotic cell division differ from the mitotic phase to support complex changes during meiosis.
Meiosis is divided into MEIOSIS I and MEIOSIS II; each meiotic phase has sequence of Prophase, Metaphase, Anaphase and Telophase.
The Prophase of Meiosis I is more significant where the primary feature of meiosis takes place: Pre – leptonema, Leptonema, Zygonema, Pachynema, Diplonema, Diakinesis and are absent in Prophase of Meiosis II.
Meiosis I
Somatic cells and germ line cells are differentiated in this method; specialized and prominent switching of process from mitotic to meiotic phase in testes and ovaries; takes place in Prophase I.
Meiosis generally skips the G2 Phase as soon as the Switch is “ON” to Prophase I.
Significant modification in the genetic material is well accounted in different phases of PROPHASE I; other cellular changes are similar in that of mitosis wherein the nuclear membrane starts disintegrating in the prophase and disintegration of other membrane bound organelles.
The specialized phases of the prophases and the events at each stage are given below:
Prophase I: Pre-Leptonema
The chromosomes are extremely thin to be identified except for the differentiated sex chromosomes which has Heter pyknotic bodies.
Heter pyknotic bodies are regions of either tightly or loosely bound chromatin fibers which are stained more or very less from the rest of the chromosomes.
Prophase I: Leptonema
o Leptonema in Greek means thin thread like structures.
o The chromosomes in the phase are characterized by thin appearance even after the replication.
o Out of all phases of meiosis the PROPHASE I is longer in all eukaryotes with time variation in different species. The phase also constitutes other changes:
o Nucleus enlarges in size occupying most of the cytoplasm signifying the increased genetic content in cells.
o Chromatin starts to form a loop of 5 – 22 µm DNA.
o DNA appear single rather than double as in mitosis because of this the phase has its name LEPTONEMA with thin chromosomal appearance.
o The chromosome starts condensing and has bead like thickened structures called chromomeres present irregularly in a chromosome and the number of chromomeres are not constant.
o Prophase chromosomes forms a telomere bouquet which orients the chromosomes theoretically to form homologous pairs.
o These telomere bouquets attach the chromosome to the inner nuclear membrane making chromosomes easier to pair.
o Synaptonemal Complexes are initiated to form in this phase as a preparatory part of next phase.
Meiosis I Prophase Diagram
Prophase I: Zygonema
o Zygonema in Greek means Adjoining. The threaded chromosomes pairs with its homologous chromosome.
o Maternal chromosome and paternal chromosome of same functions are segregated and paired to each other for recombination in next phases.
o Chromatin loops concentrate further in ZYGONEMA.
o To make sure the homologous pairing corresponds with the similar DNA sequences in both homologs.
o Recombination Complex breaks the double strand at specific sites and join the similar part of the chromosome.
o This process takes place before the synapsis of the 2 chromosomes called presynaptic complex.
o Chromosomal pairing by the synaptonemal complex.
o The homologous chromosomes pairing causes the formation of synapsis.
Synaptonemal Complex
o Synaptonemal Complex is a highly complexed structure involving proteins similar to histones to form rail road like or zipper like filaments across both chromosomes along their peripheral axis.
o Synaptonemal Complex has two kinds of filaments two Lateral and 2 transverse or Medial filaments.
o Synaptonemal Complex prevents the complete fusion of homologous chromosome with 100nm gap.
o The initiation of the complex is random and starts at any point of the pair; guided by the telomere bouquet attachment to the inner nuclear membrane.
o At the point of attachment of telomere to the nuclear membrane the Synaptonemal Complex deposits to form a thick fixation plate.
o These fixation plate attracts the re – formation of nuclear pore annuli at the region of attachment.
o For a homologous chromosome to form the process must be made possible by multitudinal involvement of various parts and process of the nuclear complex and Synaptonemal Complex forms a skeleton to support the complex formation and recombination of the chromosome providing stability.
Prophase I: Pachynema
o Pachynema refers to thick chromosome in Greek. The stage is significant because of the crossing over and recombination of the maternal and paternal chromosome results in genetically different species.
o SC is complete
o Chromatin loop is well concentrated making the genetic material to have brush like appearance.
o The number of chromosomes reduces to half forming bivalents or tetrads.
o The region of connection between the bivalents are termed as chiasma where the homologs forms X – shaped connection to hold each paternal and maternal chromosome.
o Chromatids of homologue becomes 8 with 8 kinetochores on each chromatid.
o SC ensures the homologous pairing of all chromosomes in the nucleus before proceeding to next process.
o The SC remains intact throughout the pachytene.
o Crossing over between homologous pairs takes place.
o Crossing over is regulated by components and are determined to provide structural support and genetic variability and diversity among the species.
o Crossing over regulations takes before crossing over ensures the chromosome to attain more than one recombination and restricts the closely related genes from crossing over.
o A separate rule prevails to conserve the integrity of the chromosome.
o The chromosome is divided into “Hotspots” and “Cold spots” based on the recombination sites.
o Telomeric and heterochromatin centromere regions are prevented from crossing over.
o Other regions are exposed to the crossing over for the recombination of the genetic materials among the maternal and paternal genes.
o Recombination also takes place in Pachytene stages indicated by the formation of Recombination Nodules which has intact SC to ensure the cuts which are produced to recombine does not eliminate the region from the chromosome which leads to errors in cell division.
o The recombination nodule forms a bar like structure across the chromatids to reach its corresponding pair and exchange its DNA material.
Prophase I: Diplonema
o The crossed over chromosomes are separated in the phase but are held by chiasmata.
o SC is removed at the stage after the crossing over.
o Chiasmata is intact and separation of paternal maternal chromosomes at most of the sites takes place.
o The removal of cross overs and recombination sites leaving a single site makes the four tetrads are visible.
o Diplonema is more significant because of long duration it takes for all chromosomes to separate.
o In certain species; the chromosomes have a specialized appearance of lamp brush.
Prophase I: Diakinesis
o Diakinesis is the terminal process of prophase I where the chromosome’s chiasmata are completely lost except at the end region, this process is the TERMINALIZATION.
o Diakinesis in Greek means breaking across; where the chromosomes are cut across each other and marks the end of Prophase I
Pro-Metaphase
o Disintegration of Nuclear membrane
o Complete condensation of chromosomes
o The kinetochores of homologous chromosomes attach to microtubules
o Sister kinetochores maintains the integrity as a functional unit.
Meiosis I Diagram
Metaphase
o Similar to mitosis Metaphase I has similar functions
o The metaphase arranges the bivalents at the equator by the microtubules
o The main difference is the chiasma between the homologous chromosome remains intact
Prophase I: Anaphase I
o The Cohesins in the chromosomal arms are removed to break free the homologous pairing and tension created because of the separation makes the microtubule to contract towards the organizing center.
o The chromosomes are segregated to the poles.
Prophase I: Telophase I
Telophase is simple and marked by formation of the nuclear membrane by furrow in animals and phragmoplasts in plants and the cell separation – Cytokinesis.
Meiosis I Citations
- SnapShot: Meiosis – Prophase I. Cell . 2020 Jun 11;181(6):1442-1442.e1.
- LINC complexes as regulators of meiosis. Curr Opin Cell Biol . 2018 Jun;52:22-29.
- Conservation and Variability of Meiosis Across the Eukaryotes. Annu Rev Genet . 2016 Nov 23;50:293-316.
- Mixing and Matching Chromosomes during Female Meiosis. Cells . 2020 Mar 12;9(3):696.
- Elevated Mutagenicity in Meiosis and Its Mechanism. Bioessays . 2019 Apr;41(4):e1800235.
- The molecular biology of meiosis in plants. Annu Rev Plant Biol . 2015;66:297-327.
- Recombination, Pairing, and Synapsis of Homologs during Meiosis. Cold Spring Harb Perspect Biol . 2015 May 18;7(6):a016626.
- Meiosis: the chromosomal foundation of reproduction. Biol Reprod . 2018 Jul 1;99(1):112-126.
- The cohesin complex in mammalian meiosis. Genes Cells . 2019 Jan;24(1):6-30.
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Significance of Mitosis: Definition, Mitotic Phase, and...
Continue ReadingSignificance of Mitosis: Introduction
Growth and development are essential in to an organism to survive in dynamic environment.
Development and growth are inevitable changes which requires and needs a specific mechanism to grow in an adverse condition and ensure a healthy progeny to do the same.
Cell division in general is the process where an organism grows and develops in the ever-changing environment.
To ensure the line of a species they reproduce in which half of parent’s genetic material is transferred to the young one by the process of reduced division of cell; plays a primary and minimal role in developing a progeny.
In an organism’s life the cell division plays a major role in development; determining the structure, function and capacity according to the genetic expression.
The patterning and molding are mediated by exponential cell division known as the mitosis.
Changes in cell division which is not inherited to the progeny but affects the regular functions are observed in Somatic cells – divides through mitosis.
Mechanism of Mitosis
Mitosis is one of the phases in a cell cycle; is a process which produces two identical daughter cells from a single parent cell; induced when proper nutrient requirement and hormonal internal cues are synchronized.
Mitosis is preceded by 2 Gap phases and single Synthesis phase where diploid cells replicate and becomes tetraploid chromosomes with 4 arms and a centromere – highly condensed region during initial mitosis stage – Late prophase and Metaphase.
The commitment to enter the cell cycle is given by the expression of protein dependent kinases – dimers in cell cycle control; Cyclin and Cyclin Dependent Kinases.
Preparatory interphase makes a cell capable of self-division and replication of cellular function from the preexisting cells.
Mitosis itself proceeds through different phases of prophase, metaphase, anaphase and telophase; arranging and segregation the chromosomes and other cell materials and organelles (precursors) from the existing cells.
Pre-Mitotic Phase
The regulatory proteins make essential measures and fulfills cell’s need to enter mitosis in a steady state.
At each stage the cells gain an important feature to undergo cell division and divide genetic and other cell component evenly to each daughter cell.
During Interphase; it is the non – dividing phase of a cell where the cells undergo synthesis of RNA, Proteins and genetic material to proceed the cycle; are frequently regulated by protein kinases phosphorylating inhibitors to repress few genes and enhance the gene expression of few others to synthesis DNA.
Checkpoints at each phase makes the cell cycle error free and maintains the integrity of the cycle.
The G1 phase has a checkpoint which when crossed; the cells get committed to mitosis irreversibly is governed by the CDK – CYC complexes in eukaryotes.
The G1 checkpoint makes sure the cell undergoing mitosis has essential nutrients to proceed throughout the cell cycle.
The synthesis phase is initiated when the CDKA – CYC D of G1 produces transcriptional factors (ORC – Origin Replication Complex) which sits on the replication origin region of the chromatin fibers and initiate DNA Replication in S Phase.
When the cell completes the S Phase, they are auto regulated by same Transcriptional factors induced G1 phase protein kinases which limits replication when the cells have an ideal DNA for mitosis.
The cells enter G2 Phase. CDKA – CYCA expression increases in G2 Phase produces transcriptional factors which completes gene expression for other cellular mitotic requirements and when the expression reaches a particular threshold activates other G2 transcriptional factors which repress the gene expression and promotes.
Condensation of the chromatin fibers and prepare the cell to enter mitosis.
Mitotic Phase
Mitosis is regulated by CYCB – CDKB from late G2 phase and are then dominated by CYCD – CDKA where each phase has their significant feature starting from prophase; the chromatin condenses to chromosome but not completely; includes the disintegration of many cell organelles and nuclear membrane and in cytoplasm many microtubules are formed.
In Plants a specialized pre – prophase band starts to form.
Metaphase marks the complete elimination of nuclear membrane and condensed chromosomes are formed.
Later the preprophase band in plants attach to the kinetochores and make the chromosome to arrange at metaphase equatorial plate after all chromosomes are arranged by the spindle fibers of opposite poles the cell enters Anaphase.
The sister chromatids re separated by breaking Cohesins binding the chromosomes by proteolysis and drags the sister chromatids to opposite poles.
In Telophase; the daughter chromatids on opposite ends starts decondensation; reappearance of Golgi complex mitochondria chloroplast and other cell organelles are formed.
The cell cycle when complete develops the cell wall and remains adjacent to each other; when induced enters the cell cycle again or remains same in the G1 phase or enter G0 phase for maturation till they are eliminated.
Significance of Mitosis
o Mitosis is the primary mechanism for an organism to grow and helps in development.
o After birth the child develops both quantitatively and qualitatively.
o Mitosis produces new cells from existing cells and increases exponentially to increase the whole growth of the organism.
o Division is from preexisting cells. Every cell in our body is a result of a division of a single cell.
o Cells which enter the cell cycle take up essential nutrients and when an organism undergoes a development or to replace older ones; an induction signal and certain modifying factors makes the cell enter the cell cycle from G0 Phase to produce new cells; is the first type.
o The other type includes ever dividing meristematic cells in plants for growth and cells of intestine and skin of animals where the dead cells must frequently be removed to protect the internal organs from direct exposure to acid and sun.
o Mitosis takes place at appropriate time and appropriate internal cues.
o All cells are not capable of cell division and enter into a G0 Phase where they remain the same all their period of life.
o A nerve cell cannot regenerate or repair any damage caused to it even with stimulus and nutrients; unlike enterocytes, skin cells in animals and meristematic cells in plants which on appropriate nutrition provide continuous growth for the plant.
o In the absence of the appropriate conditions the plant parts under the condition become dormant or temporarily stopped for a particular time and develop when appropriate conditions rise.
o Protects the body from errors of DNA Replication.
o The checkpoints of cell division at each stage of cell cycle from G1 Phase checks primary availability of all essential components such as nutrients, DNA defects, cell size and hormonal influence before the entry of cell into cell cycle.
o G2 phase ensures the health of DNA and DNA replication in Synthesis phase when such defects are noticed, cell cycle halts and repair mechanisms for damaged DNA or extend time for complete DNA Replication.
o Replenish the tissue cell pool and organization.
o In animals’ specific tissues exposed for heavy work are subjected to wear and tear; makes the cell lose its ability to continue the role and efficiency is reduced.
o For Example: Red Blood Cells carrying Oxygen to the tissues, Enterocyte’s exposure to acidic bolus, Stem cells, Skin cell’s exposure to external mechanical stressors such as sun light, pressure etc., requires set of cells to be exposed to and these regions are given the genetic activation of continuous cell division to maintain the tissue pool and sufficient function.
o Provides structural integrity and stability to an organism.
o Cells make up the whole organismal existence which must develop over a period of time to maintain its structure and shape for proper function and survival of the organism.
o A proper cell function is essential to maintain a Homeostasis.
o A small malfunction in a cell caused by external source factors might disrupt the homeostasis causing deviation from the ideal cell.
o These deviations are identified and rectified by the checkpoints of the cell cycle.
o Mitosis is conserved over evolution in eukaryotes.
o The mechanism of how a cell divide is the same in all eukaryotes.
o The main difference will be the signals, nutrients and the proteins involved in regulation of cell cycle.
o Ratio between the cytoplasm and nucleoplasm are maintained.
o The cytoplasm and nucleoplasm are derived from similar components and have the same function of storing components for cell functioning.
o The quantity and few other features such as viscosity differs in cells are maintained during the mitosis at the end of cycle.
o The ratios must be maintained constantly where the deviations will make the cell dysfunctional.
Defects of Mitosis
o Cell division in somatic cells is not inherited by progeny but can be fatal when the regulatory functions does not work properly.
o Most of the time when a cell encounters a somatic DNA errors the body has developed many mechanisms to eliminate the divergent to maintain the homeostasis of a system.
o But the errors when not detected might lead to tumor and cancer which is fatal.
o Reasons for errors in cell division is not clearly known; it might be a result of mutation caused by exposure to carcinogenic gasses, X – rays or other chemical substances.
o Defects of mitosis in plants leads to aneuploidy and does not have fatal effects as in animals and humans.
Few defects in Mitosis are:
1. SAC (Spindle Assembly Checkpoint) defects
2. Cohesins defect
3. Merotelic Attachments
4. K – MT Stability
5. Centrosome amplification
6. Tetra ploidy
Most of the defects are common and are predominantly found in animals.
Impacts in plants are usually less and mostly results in aneuploidy; which is not inherited to the off springs and not fatal.
Significance of Mitosis Citations
- Mitosis in filamentous fungi: how we got where we are. Fungal Genet Biol . 1999 Jun;27(1):1-25.
- Abnormal mitosis in reactive astrocytes. Acta Neuropathol Commun . 2020 Apr 15;8(1):47.
- Mitosis in vertebrates: the G2/M and M/A transitions and their associated checkpoints. Chromosome Res . 2011 Apr;19(3):291-306.
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Cytokinesis: Definition, Checkpoints, Diagram, and Examples
Continue ReadingCell Cycle, Cell Cycle Phases, and Cytokinesis
Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.
Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.
The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues. Cell cycle is a sequential development of cell between two cell divisions.
The cycle is genetically controlled and are programmed in every cell and are specific for each region.
Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.
Two Phases of cell cycle are: INTERPHASE and MITOTIC PHASE. INTERPHASE involves G1, synthesis and G2 phase; chromosomal replication and development is regulated by the phase; determines the quality and quantity of chromosome entering the daughter cells and a balance is maintained by the phase.
Karyokinesis and Cytokinesis division, segregation of chromosome and cell takes place during MITOTIC PHASE.
The notable feature of cell cycle is eukaryotic organisms though diverse and distinct have a common type of cell division over the Kingdom of Eukaryotes is a scientific wonder and research have emphasized that timing of a cell entering the cell cycle is essential in cell cycle regulation.
History of Cell Cycle
The history of mitosis dates back to 18th and 19th century where the aid from microscope and visibility of cell division under the microscope supported the discovery of the cell division.
The discovery was earlier than the DNA discovery was a breakthrough in the scientific community as it answered the most intriguing question of Humans “How do we grow? Develop? Reproduce? What is the driving factor for the growth? How do we resemble our parents?” etc.,
Before the discovery of cell division there were many theories on how the cells are related to the overall development of an organism’s lifecycle.
One of which was Rudalph Virchow’s theory of cell: “Omnis cellula e cellula” which states that a cell originates from a pre-existing cell.
Walther Flemming discovered and published a detailed book on cell division in 1882 after discovering cell division in 1879.
He named it “Mitosis” after the Greek word Mito – “Wrapping thread” owing to the thread like appearances of the chromosomes.
He conducted a detailed study and deduced staining techniques to understand cell cycle and named the stages of each division as Prophase, metaphase, Anaphase and Telophase.
The cell division was identified in Salamander’s embryo. Flemming supported Virchow’s cell theory with precision and stated that “Omnis nucleus e nucleo” which states that a nucleus origin from a pre – existing nucleus and highlighting the chromosomal segregation and laid foundation for the theory of inheritance where the chromosomes play an important role carrying the genetic information from the parent to the offspring.
Cell Cycle Phases
The cell cycle is common for all eukaryotic organisms; travelling through 2 major phases based on the cell division:
INTERPHASE and MITOTIC PHASE. Interphase consists of 3 phases Gap 1 phase, Synthesis Phase and Gap 2 Phase.
Similarly, mitosis has four phases Prophase, metaphase, anaphase, and telophase.
The development of cells through these phases are influenced and facilitated by heterodimeric protein kinases – Cyclin and Cyclin Dependent Kinases.
Mitotic Phase
The changes in above phases are minimal or not clearly visible in microscopes whereas the changes in M Phase are easily detectable.
The phase has 4 parts in which the division takes place systematically and continuously.
The cell stages are easily visible in plant parts as the specialized dividing region – MERISTEM is prevalent in roots and shoots are continuously dividing providing a mechanical support and functional integrity to the plants.
The 4 phases are: Prophase, Metaphase, Anaphase and Telophase.
Each phase has a distinctive change to be identified and Eukaryotic cells replicates in the same order in most of the organisms.
Prophase: The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division. RNA synthesis is inhibited.
Metaphase: Nuclear membrane is eliminated completely chromosomes are completely condensed. The cytoskeleton – spindle fibers attach to the kinetochores. The chromosomes are aligned in the equatorial plate.
Anaphase: Chromosomal split forms daughter chromatids; travels to the opposite poles. The chromosomes are V – Shaped as they are dragged to the opposite sites.
Telophase: Microtubules disappear and chromosomes decondense to chromatin mass. Nuclear envelope starts to form. The disintegrated organelles form again.
Cytokinesis
Next part of cell cycle is Cytokinesis where the duplicated sister chromosome is designated a separate functional unit by the formation of microtubules to form the rigid cell membrane.
Cytokinesis in Higher Plants
Cell division is complete when the daughter chromatids are segregated and are given the status of independent functioning.
This is achieved by cell cleavage and cell wall formation of the dividing cells; marked by cytokinesis.
Cell formation from the previous cells and the division of nucleus and chromosome segregation is known as karyokinesis.
Cytokinesis completion is required for a cell to attain maturity by entering the Interphase of the next cell cycle.
Predominant transfer of membrane bound organelles. But the initiation of the phase takes place in late anaphase and in Telophase with Preprophase Band (PPB). Contrast to animals; plants lack Microtubule Organizing centers (MTOC) – centrosomes which is supported by PPB formed at the median plate perpendicular to the equatorial plate.
Tubulins and Dynein’s segregates chromosomes; whereas Actin filaments guide the cell wall formation between the cells as cell division predominantly produces daughter cells in adjacent sides.
Cell Wall formation: Phragmoblast
Cell wall formation separates the daughter cells is the main event in CYTOKINESIS.
Cellulosic cell wall formation is semi – rigid and are guided by PHRAGMOPLASTS.
Phragmoplasts arise from the PPB after Telophase formed by the interzonal or interpolar microtubules; at the middle region of the cell will use microtubules and Golgi vesicles to form a cell plate.
The cell plate formation takes place centrifugally referred as Nascent Cell Plate.
The nascent cell plate forms the initial semi rigid cell wall extends and reach the adjacent cell wall from the center.
Cell communication becomes essential in a tissue where the availability of the nutrients and other information’s are passed through certain pores namely Plasmodesmata are formed by disintegrated Golgi vesicles carrying pectin.
Pectin of one vesicle when fuses with pectin of other vesicle they components mix and forms pores.
These early cells plate / nascent plate formation is supported by Golgi vesicles which contains glycoproteins and polysaccharides for the cell wall formation.
The microtubules forms semi – crystalline lattice on both side of the daughter cell.
Later they mature to form a rigid cell wall by G1 PHASE.
Segregation of Membrane Bound Organelles
By the time of cell division (i.e.) Cytokinesis the cell organelles are equally divided among the daughter cells.
The cell organelles which are bound by a membrane are transported to the cells by motor proteins to the poles.
Generally, mitochondria and chloroplasts are many in number which is sufficient for each cell is multiplied during the mitosis and transported before cytokinesis.
Endoplasmic reticulum which is a part of nucleus cuts off during cell plate formation to the daughter cell.
The chromatids completely condense and are bound by nuclear membrane.
Cytokinesis Citations
- Cytokinesis and cancer. FEBS Lett . 2010 Jun 18;584(12):2652-61.
- Update on plant cytokinesis: rule and divide. Curr Opin Plant Biol . 2019 Dec;52:97-105.
- Positioning cytokinesis. Genes Dev . 2009 Mar 15;23(6):660-74.
- Cytokinesis in trypanosomes. Cytoskeleton (Hoboken) . 2012 Nov;69(11):931-41.
- Mechanics of cell division and cytokinesis. Mol Biol Cell . 2018 Mar 15;29(6):685-686.
- Cytokinesis defects and cancer. Nat Rev Cancer . 2019 Jan;19(1):32-45.
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Telophase: Definition, Checkpoints, Diagram, and Examples
Continue ReadingCell Cycle, Cell Cycle Phases, and Telophase
Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.
Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.
The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues. Cell cycle is a sequential development of cell between two cell divisions.
The cycle is genetically controlled and are programmed in every cell and are specific for each region.
Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.
Two Phases of cell cycle are: INTERPHASE and MITOTIC PHASE. INTERPHASE involves G1, synthesis and G2 phase; chromosomal replication and development is regulated by the phase; determines the quality and quantity of chromosome entering the daughter cells and a balance is maintained by the phase.
Karyokinesis and Cytokinesis division, segregation of chromosome and cell takes place during MITOTIC PHASE.
The notable feature of cell cycle is eukaryotic organisms though diverse and distinct have a common type of cell division over the Kingdom of Eukaryotes is a scientific wonder and research have emphasized that timing of a cell entering the cell cycle is essential in cell cycle regulation.
History of Cell Cycle
The history of mitosis dates back to 18th and 19th century where the aid from microscope and visibility of cell division under the microscope supported the discovery of the cell division.
The discovery was earlier than the DNA discovery was a breakthrough in the scientific community as it answered the most intriguing question of Humans “How do we grow? Develop? Reproduce? What is the driving factor for the growth? How do we resemble our parents?” etc.,
Before the discovery of cell division there were many theories on how the cells are related to the overall development of an organism’s lifecycle.
One of which was Rudalph Virchow’s theory of cell: “Omnis cellula e cellula” which states that a cell originates from a pre-existing cell.
Walther Flemming discovered and published a detailed book on cell division in 1882 after discovering cell division in 1879.
He named it “Mitosis” after the Greek word Mito – “Wrapping thread” owing to the thread like appearances of the chromosomes.
He conducted a detailed study and deduced staining techniques to understand cell cycle and named the stages of each division as Prophase, metaphase, Anaphase and Telophase.
The cell division was identified in Salamander’s embryo. Flemming supported Virchow’s cell theory with precision and stated that “Omnis nucleus e nucleo” which states that a nucleus origin from a pre – existing nucleus and highlighting the chromosomal segregation and laid foundation for the theory of inheritance where the chromosomes play an important role carrying the genetic information from the parent to the offspring.
Cell Cycle Phases
The cell cycle is common for all eukaryotic organisms; travelling through 2 major phases based on the cell division:
INTERPHASE and MITOTIC PHASE. Interphase consists of 3 phases Gap 1 phase, Synthesis Phase and Gap 2 Phase.
Similarly, mitosis has four phases Prophase, metaphase, anaphase, and telophase.
The development of cells through these phases are influenced and facilitated by heterodimeric protein kinases – Cyclin and Cyclin Dependent Kinases.
Mitotic Phase
The changes in above phases are minimal or not clearly visible in microscopes whereas the changes in M Phase are easily detectable.
The phase has 4 parts in which the division takes place systematically and continuously.
The cell stages are easily visible in plant parts as the specialized dividing region – MERISTEM is prevalent in roots and shoots are continuously dividing providing a mechanical support and functional integrity to the plants.
The 4 phases are: Prophase, Metaphase, Anaphase and Telophase.
Each phase has a distinctive change to be identified and Eukaryotic cells replicates in the same order in most of the organisms.
Prophase: The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division. RNA synthesis is inhibited.
Metaphase: Nuclear membrane is eliminated completely chromosomes are completely condensed. The cytoskeleton – spindle fibers attach to the kinetochores. The chromosomes are aligned in the equatorial plate.
Anaphase: Chromosomal split forms daughter chromatids; travels to the opposite poles. The chromosomes are V – Shaped as they are dragged to the opposite sites.
Telophase: Microtubules disappear and chromosomes decondense to chromatin mass. Nuclear envelope starts to form. The disintegrated organelles form again.
Characteristic of Telophase
The phase starts when the equal segregation of chromatids takes place in the daughter cells and the lost organelles from the nuclear envelope to other organelles are re – formed as the phosphorylated proteins are still intact and evenly distributed among the cells.
The mechanism involves dephosphorylation of the CDK – CYC complexes to do following activity:
1. De-condensation of chromosomes to chromatin allowing gene transcription
2. Nuclear envelope formation along with lamina – Dephosphorylation. Initially after the segregation and separation the nuclear envelope precursor is said to be around chromosomes which on dephosphorylation gradually develops nuclear envelope.
3. Nuclear pore proteins are embedded for the transport of nucleus essential material
4. Endoplasmic Reticulum formation intact to the nucleus.
5. Separation of Microtubules.
Following this Further inactivation of CDK-CYC complex led the cells to prepare for next cycle’s interphase.
Telophase Citations
- Aurora B kinases restrict chromosome decondensation to telophase of mitosis. Cell Cycle . 2008 Feb 1;7(3):293-6.
- DNA double-strand breaks in telophase lead to coalescence between segregated sister chromatid loci. Nat Commun . 2019 Jun 28;10(1):2862.
- The telophase disc: its possible role in mammalian cell cleavage. Bioessays . 1993 Mar;15(3):201-7.
- A journey with common fragile sites: From S phase to telophase. Genes Chromosomes Cancer . 2019 May;58(5):305-316.
- The DNA Damage Response in Telophase: Challenging Dogmas. Bioessays . 2020 Jul;42(7):e2000085.
- A chromosome folding intermediate at the condensin-to-cohesin transition during telophase. Nat Cell Biol . 2019 Nov;21(11):1393-1402.
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Metaphase: Definition, Checkpoints, Diagram, and Examples
Continue ReadingCell Cycle, Cell Cycle Phases, and Metaphase
Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.
Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.
The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues. Cell cycle is a sequential development of cell between two cell divisions.
The cycle is genetically controlled and are programmed in every cell and are specific for each region.
Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.
Two Phases of cell cycle are: INTERPHASE and MITOTIC PHASE. INTERPHASE involves G1, synthesis and G2 phase; chromosomal replication and development is regulated by the phase; determines the quality and quantity of chromosome entering the daughter cells and a balance is maintained by the phase.
Karyokinesis and Cytokinesis division, segregation of chromosome and cell takes place during MITOTIC PHASE.
The notable feature of cell cycle is eukaryotic organisms though diverse and distinct have a common type of cell division over the Kingdom of Eukaryotes is a scientific wonder and research have emphasized that timing of a cell entering the cell cycle is essential in cell cycle regulation.
History of Cell Cycle
The history of mitosis dates back to 18th and 19th century where the aid from microscope and visibility of cell division under the microscope supported the discovery of the cell division.
The discovery was earlier than the DNA discovery was a breakthrough in the scientific community as it answered the most intriguing question of Humans “How do we grow? Develop? Reproduce? What is the driving factor for the growth? How do we resemble our parents?” etc.,
Before the discovery of cell division there were many theories on how the cells are related to the overall development of an organism’s lifecycle.
One of which was Rudalph Virchow’s theory of cell: “Omnis cellula e cellula” which states that a cell originates from a pre-existing cell.
Walther Flemming discovered and published a detailed book on cell division in 1882 after discovering cell division in 1879.
He named it “Mitosis” after the Greek word Mito – “Wrapping thread” owing to the thread like appearances of the chromosomes.
He conducted a detailed study and deduced staining techniques to understand cell cycle and named the stages of each division as Prophase, metaphase, Anaphase and Telophase.
The cell division was identified in Salamander’s embryo. Flemming supported Virchow’s cell theory with precision and stated that “Omnis nucleus e nucleo” which states that a nucleus origin from a pre – existing nucleus and highlighting the chromosomal segregation and laid foundation for the theory of inheritance where the chromosomes play an important role carrying the genetic information from the parent to the offspring.
Cell Cycle Phases
The cell cycle is common for all eukaryotic organisms; travelling through 2 major phases based on the cell division:
INTERPHASE and MITOTIC PHASE. Interphase consists of 3 phases Gap 1 phase, Synthesis Phase and Gap 2 Phase.
Similarly, mitosis has four phases Prophase, metaphase, anaphase, and telophase.
The development of cells through these phases are influenced and facilitated by heterodimeric protein kinases – Cyclin and Cyclin Dependent Kinases.
Mitotic Phase
The changes in above phases are minimal or not clearly visible in microscopes whereas the changes in M Phase are easily detectable.
The phase has 4 parts in which the division takes place systematically and continuously.
The cell stages are easily visible in plant parts as the specialized dividing region – MERISTEM is prevalent in roots and shoots are continuously dividing providing a mechanical support and functional integrity to the plants.
The 4 phases are: Prophase, Metaphase, Anaphase and Telophase.
Each phase has a distinctive change to be identified and Eukaryotic cells replicates in the same order in most of the organisms.
Prophase: The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division. RNA synthesis is inhibited.
Metaphase: Nuclear membrane is eliminated completely chromosomes are completely condensed. The cytoskeleton – spindle fibers attach to the kinetochores. The chromosomes are aligned in the equatorial plate.
Anaphase: Chromosomal split forms daughter chromatids; travels to the opposite poles. The chromosomes are V – Shaped as they are dragged to the opposite sites.
Telophase: Microtubules disappear and chromosomes decondense to chromatin mass. Nuclear envelope starts to form. The disintegrated organelles form again.
Metaphase
Metaphase events are not clearly deduced but it mainly concerns in the arrangement of the chromatids in the equatorial plane for even distribution of the genetic material among daughter cells.
The Kinetochore – Spindle fiber complex does not hold the chromosome at equatorial plate.
The process of aligning chromatids at a single plate is the congression.
The mechanism of congression is still unknown and so remains the initiation of METAPHASE.
Metaphase uses GTP to maintain a steady state and end of the phase is a transitory period and essential point in cell division is well known to dephosphorylations induces changes in cell regulators – CYC – CDK complex and initiates APC\C.
Pro-Metaphase
Chromosomal condensation completes in prometaphase.
Chromosomal segregation is initiated in this phase by the attachment of spindle fibers from centrosomes in animal cells as plant cell lacks centrosomes.
Cellulosic cell wall of plants does not make the cell dynamic to organized into spherical shape for the sister chromatids to produce into a new cell; plants organize a special band called Prophase band (PPB) perpendicular to the site of equatorial plate formation of chromosomes.
The PPB consists of actin and other microtubules coordinates with kinetochores and chromatin to assemble the chromosomes in the metaphase plate for segregation.
Additional microtubules arise just outside of the nuclear membrane after complete disintegration in early prometaphase.
Attachment of spindle fibers are made sure in prometaphase and are checked by checkpoints and relies on spindle attachment techniques and prerequisites of spindle formation.
Metaphase, Spindle Fibers and Functions
Spindle fibers are connecting machines contributes to the mechanical support to the cellular dynamic and preserves structural integrity of a cell.
Spindle fibers are protein complexes formed and named by the protein such as Dynein, Kinesin, Actin, Tubulin etc., are varied among the living system; has an indispensable role in ensuring the survival.
The fibers are nucleated and attach to the kinetochores contributes to the cellular dynamics and mitotic dynamic (i.e.) chromosomal segregation and shape perseverance.
The spindle fibers are bipolar in origin from centrosomes of animals and PPB in plants associates with kinetochores by specific mechanism of “Search and Capture”
Oscillation of microtubules in the cytoplasm in search of kinetochores of the chromosomes from opposite sides to hold them in the mitotic plate and pass to anaphase.
The spindle and the chromosomes are dynamic structures which constantly changes its position.
The kinetochores must be appropriately attached to its side and the opposite sister chromatid kinetochore must bind to the opposite pole to ensure equal distribution of the genetic material.
The microtubule when attached to a kinetochore many other microtubules branching from the existing microtubule or formation from the kinetochore attaches along the existing microtubule forming spindle fiber.
The spindle fiber attachment is succeeded by CDK – CYC complex and other supporting proteins such as Augnin in animals or generally a RAN – GTP associated pathway ensures the connection of chromosomes.
SAC, Metaphase, and Spindle Fibers
Spindle Assembly Checkpoint (SAC) is the main gateway for a cell to cross the metaphase supported by the kinases of kinetochores.
The kinases ensure attachment of all chromatids to its respective poles.
Kinetochore’s orientation is an initial property where it ensures a proper attachment to the respective spindle fibers by their bipolar coordination.
The spindle fibers arise from both poles to capture chromatids are driven by motor present in kinetochore on binding tends to direct the chromatids to the point of origin.
The mitotic spindle attachment creates an active motility with the spindle fibers and kinetochores and becomes unstable.
Stability is attained when the sister kinetochore attaches to the opposite microtubule which creates a tension enabling chromatids to oscillate from a pole to other.
SAC as said involves kinases to ensure the chromatid connectivity to proceed to next phase is regulated by a cascade.
The main principle is that the attached kinetochores support Anaphase promoting complex or cyclosomes to proceed to the next phase.
When a polyubiquitination by an APC/C are inhibited the cell division is temporarily held back to ensure the connection of the kinetochore which completes the cascade and ensures the entry to ANAPHASE.
Metaphase Citations
- The metaphase chromosome as a reporter of nuclear activity. Exp Cell Res . 1996 Dec 15;229(2):198-200.
- Microtubules Enhance Mesoscale Effective Diffusivity in the Crowded Metaphase Cytoplasm. Dev Cell . 2020 Sep 14;54(5):574-582.e4.
- Classification of Metaphase Chromosomes Using Deep Convolutional Neural Network. J Comput Biol . 2019 May;26(5):473-484.
- Active forces shape the metaphase spindle through a mechanical instability. Proc Natl Acad Sci U S A . 2020 Jul 14;117(28):16154-16159.
- A review of metaphase chromosome image selection techniques for automatic karyotype generation. Med Biol Eng Comput . 2016 Aug;54(8):1147-57.
- The Physics of the Metaphase Spindle. Annu Rev Biophys . 2018 May 20;47:655-673.
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Anaphase: Definition, Checkpoints, Diagram, and Examples
Continue ReadingCell Cycle, Cell Cycle Phases, and Anaphase
Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.
Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.
The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues. Cell cycle is a sequential development of cell between two cell divisions.
The cycle is genetically controlled and are programmed in every cell and are specific for each region.
Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.
Two Phases of cell cycle are: INTERPHASE and MITOTIC PHASE. INTERPHASE involves G1, synthesis and G2 phase; chromosomal replication and development is regulated by the phase; determines the quality and quantity of chromosome entering the daughter cells and a balance is maintained by the phase.
Karyokinesis and Cytokinesis division, segregation of chromosome and cell takes place during MITOTIC PHASE.
The notable feature of cell cycle is eukaryotic organisms though diverse and distinct have a common type of cell division over the Kingdom of Eukaryotes is a scientific wonder and research have emphasized that timing of a cell entering the cell cycle is essential in cell cycle regulation.
History of Cell Cycle
The history of mitosis dates back to 18th and 19th century where the aid from microscope and visibility of cell division under the microscope supported the discovery of the cell division.
The discovery was earlier than the DNA discovery was a breakthrough in the scientific community as it answered the most intriguing question of Humans “How do we grow? Develop? Reproduce? What is the driving factor for the growth? How do we resemble our parents?” etc.,
Before the discovery of cell division there were many theories on how the cells are related to the overall development of an organism’s lifecycle.
One of which was Rudalph Virchow’s theory of cell: “Omnis cellula e cellula” which states that a cell originates from a pre-existing cell.
Walther Flemming discovered and published a detailed book on cell division in 1882 after discovering cell division in 1879.
He named it “Mitosis” after the Greek word Mito – “Wrapping thread” owing to the thread like appearances of the chromosomes.
He conducted a detailed study and deduced staining techniques to understand cell cycle and named the stages of each division as Prophase, metaphase, Anaphase and Telophase.
The cell division was identified in Salamander’s embryo. Flemming supported Virchow’s cell theory with precision and stated that “Omnis nucleus e nucleo” which states that a nucleus origin from a pre – existing nucleus and highlighting the chromosomal segregation and laid foundation for the theory of inheritance where the chromosomes play an important role carrying the genetic information from the parent to the offspring.
Cell Cycle Phases
The cell cycle is common for all eukaryotic organisms; travelling through 2 major phases based on the cell division:
INTERPHASE and MITOTIC PHASE. Interphase consists of 3 phases Gap 1 phase, Synthesis Phase and Gap 2 Phase.
Similarly, mitosis has four phases Prophase, metaphase, anaphase, and telophase.
The development of cells through these phases are influenced and facilitated by heterodimeric protein kinases – Cyclin and Cyclin Dependent Kinases.
Mitotic Phase
The changes in above phases are minimal or not clearly visible in microscopes whereas the changes in M Phase are easily detectable.
The phase has 4 parts in which the division takes place systematically and continuously.
The cell stages are easily visible in plant parts as the specialized dividing region – MERISTEM is prevalent in roots and shoots are continuously dividing providing a mechanical support and functional integrity to the plants.
The 4 phases are: Prophase, Metaphase, Anaphase and Telophase.
Each phase has a distinctive change to be identified and Eukaryotic cells replicates in the same order in most of the organisms.
Prophase: The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division. RNA synthesis is inhibited.
Metaphase: Nuclear membrane is eliminated completely chromosomes are completely condensed. The cytoskeleton – spindle fibers attach to the kinetochores. The chromosomes are aligned in the equatorial plate.
Anaphase: Chromosomal split forms daughter chromatids; travels to the opposite poles. The chromosomes are V – Shaped as they are dragged to the opposite sites.
Telophase: Microtubules disappear and chromosomes decondense to chromatin mass. Nuclear envelope starts to form. The disintegrated organelles form again.
Types of Anaphase
Anaphase has 2 phases Anaphase A and Anaphase B were the segregation and spindle fiber withdrawal happen respectively.
Chromosomes separates to chromatids by breaking the wounding of COHESIN which holds the chromosome intact in the metaphase; is Anaphase A.
I. Anaphase A
Anaphase A involves the cell cycle regulator proteins of the M phase.
The anaphase elucidation is poorly studied for plants rather than animals; but the cell cycle and the mechanism is conserved over evolution of the living organism.
It can give a basic picture of the mechanism which is similar to all eukaryotes were the difference is the proteins and other components involved in them governed by the external cues and time.
The mechanism is a cascade of proteolysis, degradation and activation different protein molecules.
The first step is the proteolysis of cohesins mediated by APC/C along with Cdc ubiquitinates CYCB – CDKB complex also degrades the Securin.
Securin directly inhibits Separase in chromosomes cleaves Cohesin rings at Scc1 segregate the chromatids.
The separation induces respective poleward movement of the chromatids.
I. Anaphase B
Anaphase B is the self-detachment of microtubules from the KMT complex due to loss of MT protein components.
Dynein and Kinesin are the 2 proteins; these are motor proteins which pulls the KMT to opposite poles especially Dynein.
The removal of microtubules is mediated by depolymerization of the proteins components leads to separation of each tubulin component from the minus driven ends and Dynein at the control of the fibers near plasma membrane.
Removal of KMT complex is simultaneous with chromatid retraction to the poles.
Motor protein Kinesin undergoes a sliding elongation forming interpolar MT at the plus end.
The main function of the interpolar MTs are to separate the poles far apart from the chromosomal segregation.
Anaphase Citations
- Detection of Ultrafine Anaphase Bridges. Methods Mol Biol . 2018;1672:495-508.
- Anaphase promoting complex or cyclosome? Cell Cycle . 2005 Nov;4(11):1585-92.
- Evidence for anaphase pulling forces during C. elegans meiosis. J Cell Biol . 2020 Dec 7;219(12):e202005179.
- Anaphase in vitro. Proc Natl Acad Sci U S A . 1996 Oct 29;93(22):12076-7.
- Engaging Anaphase Catastrophe Mechanisms to Eradicate Aneuploid Cancers. Mol Cancer Ther . 2018 Apr;17(4):724-731.
- Force-generating mechanisms of anaphase in human cells. J Cell Sci . 2019 Sep 16;132(18):jcs231985.
- Anaphase: a fortune-teller of genomic instability. Curr Opin Cell Biol . 2018 Jun;52:112-119.
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Prophase: Definition, Checkpoints, Diagram, and Examples
Continue ReadingCell Cycle, Cell Cycle Phases, and Prophase
Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.
Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.
The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues. Cell cycle is a sequential development of cell between two cell divisions.
The cycle is genetically controlled and are programmed in every cell and are specific for each region.
Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.
Two Phases of cell cycle are: INTERPHASE and MITOTIC PHASE. INTERPHASE involves G1, synthesis and G2 phase; chromosomal replication and development is regulated by the phase; determines the quality and quantity of chromosome entering the daughter cells and a balance is maintained by the phase.
Karyokinesis and Cytokinesis division, segregation of chromosome and cell takes place during MITOTIC PHASE.
The notable feature of cell cycle is eukaryotic organisms though diverse and distinct have a common type of cell division over the Kingdom of Eukaryotes is a scientific wonder and research have emphasized that timing of a cell entering the cell cycle is essential in cell cycle regulation.
History of Cell Cycle
The history of mitosis dates back to 18th and 19th century where the aid from microscope and visibility of cell division under the microscope supported the discovery of the cell division.
The discovery was earlier than the DNA discovery was a breakthrough in the scientific community as it answered the most intriguing question of Humans “How do we grow? Develop? Reproduce? What is the driving factor for the growth? How do we resemble our parents?” etc.,
Before the discovery of cell division there were many theories on how the cells are related to the overall development of an organism’s lifecycle.
One of which was Rudalph Virchow’s theory of cell: “Omnis cellula e cellula” which states that a cell originates from a pre-existing cell.
Walther Flemming discovered and published a detailed book on cell division in 1882 after discovering cell division in 1879.
He named it “Mitosis” after the Greek word Mito – “Wrapping thread” owing to the thread like appearances of the chromosomes.
He conducted a detailed study and deduced staining techniques to understand cell cycle and named the stages of each division as Prophase, metaphase, Anaphase and Telophase.
The cell division was identified in Salamander’s embryo. Flemming supported Virchow’s cell theory with precision and stated that “Omnis nucleus e nucleo” which states that a nucleus origin from a pre – existing nucleus and highlighting the chromosomal segregation and laid foundation for the theory of inheritance where the chromosomes play an important role carrying the genetic information from the parent to the offspring.
Cell Cycle Phases
The cell cycle is common for all eukaryotic organisms; travelling through 2 major phases based on the cell division:
INTERPHASE and MITOTIC PHASE. Interphase consists of 3 phases Gap 1 phase, Synthesis Phase and Gap 2 Phase.
Similarly, mitosis has four phases Prophase, metaphase, anaphase, and telophase.
The development of cells through these phases are influenced and facilitated by heterodimeric protein kinases – Cyclin and Cyclin Dependent Kinases.
Mitotic Phase
The changes in above phases are minimal or not clearly visible in microscopes whereas the changes in M Phase are easily detectable.
The phase has 4 parts in which the division takes place systematically and continuously.
The cell stages are easily visible in plant parts as the specialized dividing region – MERISTEM is prevalent in roots and shoots are continuously dividing providing a mechanical support and functional integrity to the plants.
The 4 phases are: Prophase, Metaphase, Anaphase and Telophase.
Each phase has a distinctive change to be identified and Eukaryotic cells replicates in the same order in most of the organisms.
Prophase: The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division. RNA synthesis is inhibited.
Metaphase: Nuclear membrane is eliminated completely chromosomes are completely condensed. The cytoskeleton – spindle fibers attach to the kinetochores. The chromosomes are aligned in the equatorial plate.
Anaphase: Chromosomal split forms daughter chromatids; travels to the opposite poles. The chromosomes are V – Shaped as they are dragged to the opposite sites.
Telophase: Microtubules disappear and chromosomes decondense to chromatin mass. Nuclear envelope starts to form. The disintegrated organelles form again.
Prophase Regulators
Onset of chromosomal condensation and differentiation is marked by the prophase requires constant expression of CYCB – CDKB complex which drives the cell division and are regulated.
The cell division is a very specific period for a developing cell which requires most of the contribution of the cell proteins and RNA to be concerned on producing two daughter cells.
Therefore, regular cell functions does not take place in the cell and whole cytoplasm will be occupied by chromosome and spindle fibers in open mitosis or the division occupies the whole nucleus with intact nuclear membrane in closed type prevalent in Yeast and lower eukaryotes.
Most of cell organelle are disintegrated at the beginning of the cycle and new spindle fibers are developed to aid in the chromosomal rearrangement genetic material segregation.
CDKB – CYCB are responsible to maintain this state throughout the cycle and other CYC – CDK complexes are phosphorylated and remains inactive; especially CDKA – CYCD whose expression might initiate growth in the mitotic cell.
Prophase Induced Cell Modification
Prophase is predominant by cellular preparation for the division includes the chromosomal condensation and disintegration of the cell organelles mainly nuclear membrane.
Prophase generally involves number of phosphorylation which are induced by unavailability of free genetic material to express the suppressing role.
These are regulated and mediated by CDK – CYC complexes of the mitotic phase where the complex induces the breakdown of nuclear membrane by phosphorylating the membrane components to breakdown and further phosphorylation of other proteins of the nucleus and the cytoplasm induce disintegration of the organelles and development of new spindle fibers.
As the cell organelles disintegrates; the mitotic spindle made of microtubules and microfilaments develops along with the chromosomal condensation by inducing the action of 2 proteins CONDENSINS and COHESIONS which coils the chromatids to chromosomes with centromeres.
Prophase Induced Nuclear Modification
I. Chromosomal Condensation
Compaction of chromosome for the ease of cellular division is aided by Condensins and Cohesions an exact mechanism on how they work is not yet achieved as perceiving chromosome is well limited by the size, integrity and complexity of it.
But theories remains that the five – subunit protein forms a ring by ATP hydrolysis and combines the chromatin and coils together.
Cohesins are similar proteins which are present from the S Phase where it binds the Replicated DNA to the sister DNA and ensures the chromatid attachment to each other.
Cohesins and Condensins belong to Structural Maintenance Protein – SMC maintains the chromosome structure and prevent them from tangle.
II. Other Events
Cell must completely prepare for the cell division to support it.
This is done by Nucleolus disintegration: condensation of the chromosomes restricts the site for gene expression which is essential for the cell coordination and synchronization.
When the region is coiled in the chromosome regular functions cease to exist thereby releasing inhibition from the nuclear membrane for CDK’s to phosphorylate the nuclear inner lamina which disintegrates into small protein speckles similar to that of ribosomes.
The condensation restricts the transcription of the cell maintenance; this may lead to the disintegration of nucleolus and nuclear envelope.
The disintegrated envelope is further displaced by Microtubules with an influx of Ca and Protein Kinase C.
The spindle formed were present form the S phase which are also eliminated and new Microtubules are formed.
Primary Constriction in the same chromosome results in the formation of a chromosomal element – centromere; plays an important role in connecting spindle fibers for the chromosomal segregation.
Prophase Induced Cytoplasmic Modification
Cell division takes place during a favorable condition where it has sufficient nutrients to promote the development of the cell cycle.
Main changes in the cell cytoplasm are: the reduced protein synthesis by a mechanism which is to be deduced.
Micro tubulars and Microfilaments from interphase disintegrate and new ones are formed, additionally; the region of cytokinesis is marked in the cell cortex by the thick actin bands – Preprophase Bands.
“Tog” domain proteins initiate the microtubular formation by catalyzing the polymerization of tubulin.
The mitotic restructure is essential to govern the cell at structural level. In mitosis; the cytoplasm remains immotile due to absence of the microtubules and microfilaments of the cell causes notable changes to support the division
1. New microtubules are synthesized which will support the cell cycle of the daughter cells.
2. The rigid cell becomes round and this promotes the even distribution of the cytoplasm and other structures to ensure the integrity of daughter cells.
3. The microtubules are soluble in the cell reduces the viscosity for ease in movement as the cytoplasm as the stream gets stationary.
All these factors ensure a proper and equal division of the cytoplasm and other organelles to the daughter cells.
Prophase Citations
- Coordinating the events of the meiotic prophase. Trends Cell Biol . 2005 Dec;15(12):674-81.
- Checkpoint mechanisms: the puppet masters of meiotic prophase. Trends Cell Biol . 2011 Jul;21(7):393-400.
- Double-strand break repair on sex chromosomes: challenges during male meiotic prophase. Cell Cycle . 2015;14(4):516-25.
- The meiotic checkpoint network: step-by-step through meiotic prophase. Cold Spring Harb Perspect Biol . 2014 Oct 1;6(10):a016675.
- The role of chromatin modifications in progression through mouse meiotic prophase. J Genet Genomics . 2014 Mar 20;41(3):97-106.
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Interphase: Definition, Checkpoints, and Examples
Continue ReadingCell Cycle and Interphase
Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.
Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.
The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues.
Cell cycle is a sequential development of cell between two cell divisions.
The cycle is genetically controlled and are programmed in every cell and are specific for each region. Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.
Two Phases of cell cycle are: INTERPHASE and MITOTIC PHASE.
INTERPHASE involves G1, synthesis and G2 phase; chromosomal replication and development is regulated by the phase; determines the quality and quantity of chromosome entering the daughter cells and a balance is maintained by the phase.
Karyokinesis and Cytokinesis division, segregation of chromosome and cell takes place during MITOTIC PHASE.
The notable feature of cell cycle is eukaryotic organisms though diverse and distinct have a common type of cell division over the Kingdom of Eukaryotes is a scientific wonder and research have emphasized that timing of a cell entering the cell cycle is essential in cell cycle regulation.
Interphase Types
Interphase of the cell cycle is non dividing phase where the cells undergo major synthesis and other integrating functions transducing signals and coordinate with other cells to produce an overall reaction to the stimulus.
Biosynthetic activity is more prevalent in this phase; absence of cell division marks a resting period; varies for every differentiated cells.
Certain cells might have a short resting period and certain cells such as neurons loses the power of differentiation and remains constant.
I. Gap1 Phase
The first gap phase starts just after the 2 daughter cells formed by mitosis is long and are species specific.
Intermediate to Mitosis and the synthesis phase the cell increases in size by synthesizing proteins and RNA for Synthesis Phase.
In a cell cycle, G1 phase is regulated by the external and internal factors.
External limiting factor is the availability of the nutrients in general eukaryotic organism and additional hormonal induction in plants.
Most of the organism restrict their cycle at G1 or enter G0, few cells on proper hormonal induction will reenter the cell cycle from G0 phase; specifically, plants have a specialized region called the meristem which continuously divide to produce cell growth.
The G1 phase determines the fate of the cell whether to enter cell cycle or to retain in G1 phase.
The cell must grow to an appropriate size and must synthesize proteins and RNA to enter S phase, and this is regulated by Cyclins and CDK’s.
II. Synthesis Phase
The threshold point – restriction point in mammals and START in other organisms; when crossed the cells enter the synthesis phase.
The G1 phase prepared the cell and its components to synthesis phase to get committed to cell division.
The cell division is arrested when external aids are less or unavailable.
The DNA replication doubles the genetic material, but the chromosome is not condensed and replicated material remains as chromatin.
Synthesis of genetic material is significant in this phase where cells are committed to divide and produce 2 daughter cells in most of the metazoans.
The S phase does not induce the increase in chromosomal numbers but doubles the genetic material.
Endoreduplication a landmark in plant endosperm is the different process of the metazoans permits DNA replication without mitosis results in ploidy. In animals, Drosophila’s salivary glands and mammal’s hepatocytes exhibit endoreplication.
Gap2 Phase
Synthesis phase and Mitosis phase is separated by G2 Phase; marked by absence of synthetic function.
This phase is a preparatory phase where the cell again enters the mitotic cycle for division.
Decondensed chromatin starts a preliminary condensation marks the start of mitotic phase.
Condensation inhibits the RNA synthesis gradually as the synthesis sites are being compactly wound for replication.
G2 Phase has minimal RNA synthesis which reduces or absent in Mitotic phase.
Interphase Checkpoints and Regulation
The cell enters cell cycle to divide and produce daughter cells or it participates in regular metabolic function is determined during the interphase.
The process becomes irreversible when the cells traverse across G1 phase and enters S phase.
Determination and commitment to cell division is regulated by protein kinases CDK – CYC complex which are conserved in plants but varies across other kingdoms; are heteromeric proteins which are predominant in eukaryotic cells determines the fate of cell based on the internal and external cues.
Each transition from stages is regulated and governed by CYC – CDK complex; involves an intricate network of mechanism involves positive and negative regulation of many numbers of components to maintain a homeostasis.
Stages of cell cycle also provides checkpoint to make sure the proper functioning of the cell cycle.
G1/S Cell Cycle Checkpoints
G1 phase is the lap phase where the cell obtains essential growth and necessary elements; increase in size by biomolecular synthesis; to proceed the cell cycle or to enter G0 to restrict them by functioning regular metabolic activities.
In most of the eukaryotes the increase in CDKA – CYCD in G1 phase makes the cell cross the restriction point and irreversibly committed to cell cycle; involves an activation pathway.
The Cyclin D concentration responds to external cues such as hormone induction, availabilities of nutrients; promote CDKA-CYCD complex thereby initiating the G1 activity; the concentration of the CYCD-CDKA complex increase the phosphorylation of Rb (Retinoblastoma) proteins.
The Rb proteins are the inhibitor of transcription factors transcribing the proteins and RNA essential for the synthesis phase.
In the absence of the CYC-CDK complex the pathway is inhibited by Rb protein.
The transcription is also inhibited when a cell synthesis the essential quantity of proteins required for S Phase or when the CYC-CDK complex over express in the process.
The transition pathway is conserved in plants. Generally; plants have a single Rb homologue and 6 types of E2F transcription factors.
E2F is further divided into 2 groups: CANONICAL and ATYPICAL. E2Fa, E2Fb, E2Fc require a dimeric substance such as DP to bind to the DNA are canonical and monomeric factors E2Fd, E2Fe, E2Ff are Atypical.
Each E2F factors has a definitive function which are not yet determined correctly; but from many scientific experiments in Arabidopsis the functions are deduced as:
E2Fa – Transcriptional
E2Fb – Transcriptional
E2Fc – Down Regulation of Transcription (Repressors)
E2Fd – unidentified
E2Fe – Prevent endocycles delaying cell elongation
E2Ff – Cell expansion Also, E2Fa and E2Fb are more essential in S Phase to balance proliferation and endoreduplication in cell.
E2F transcription leads to initiation and increase in the DNA replication by setting up a replication origin where ORC – origin replication complex binds to produce the replicating fork for DNA synthesis.
The regulating checkpoint in the process is the expression of E2Fc transcriptional factors which are induced by the genetic expression mediated by E2Fa/b; inhibit the over expression of DNA Replication.
G2/M Cell Cycle Checkpoints
The transition of the particular phase involves many genes and transcriptional factors in cycle progression.
The transcriptional factors are of 5 types: MYB3R1, MYB3R2, MYB3R3, MYB3R4 and MYB3R5.
The G2/M transition involves proteins which are also present in M phase induces cytokinesis.
The functions of transcriptional Factors are:
MYB3R2 – circadian clock
MYB3R1, MYB3R4 – activates the gene expression
MYB3R3, MYB3R5 – repress the gene expression
MYB3R3 and 5 regulates the function of gene expression of G2/M phase when it over express.
Along with this; APC/C – Anaphase promoting complex/Cyclosomes targets the proteolysis which regulates the G2/M transition and supports Mitosis phase of cell proliferation.
Interphase Citations
- Functional organisation of the genome during interphase. Curr Opin Genet Dev . 2007 Oct;17(5):451-5.
- Interphase cytogenetics and its role in molecular diagnostics of solid tumors. Am J Pathol . 1998 Feb;152(2):325-7.
- Interphase cytogenetics: at the interface of genetics and morphology. Anal Cell Pathol . 1999;19(1):3-6.
- Chromosome topology in mammalian interphase nuclei. Exp Cell Res . 1991 Feb;192(2):325-32.
- Two-step interphase microtubule disassembly aids spindle morphogenesis. BMC Biol . 2018 Jan 23;16(1):14.
- Interphase microtubules: chief casualties in the war on cancer? Drug Discov Today . 2014 Jul;19(7):824-9.
- Interphase Cell Cycle Staging using Deep Learning. Annu Int Conf IEEE Eng Med Biol Soc . 2020 Jul;2020:1432-1435.
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Cell Cycle: Definition, Description, Stages, and Checkpoints
Continue ReadingCell Cycle Introduction
Beings of earth are programmed to be born, grow, reproduce, and die. Multicellular eukaryotes develop from a unit cell to an array of trillion celled creature by a simple mechanism of cell division.
The capacity to divide is the measure of growth of an organism and the capacity to grow differs for every species are limited by internal and external factors.
Proliferation of cell can be infinite which might have resulted in an overwhelming cornucopia of cells.
The cell proliferation and growth are finite and regulated by an organism by spatial and temporal means according to its functional ability.
The increase in cell corresponds to the growth an organism exhibits; regulated or stopped after a particular period.
Plants tends to show a regulated unlimited growth; developing over lifetime a distinctive character of plants to support the sedentary life; but the growth is governed by internal and external cues of development.
Specialized regions such as meristem plays a primary role in plant cell division and growth.
Growth is regulated at cellular level by the cell cycle and growth regulators plays a significant role in inducing growth and supporting the structural composition of growth.
Periodic stages of cell cycle involve the major function of a single cell over a lifetime following a repetitive phase of development and function.
What is Cell Cycle
Cell cycle is the sequential process taking place to regulate the growth of organism; cell divides to produce a genetic replica and enters the stage of cell growth.
Cell growth involves the synthesis of organic material and integrates information across its counter parts for synchronous development of the whole body.
The cell synthesis phase lasts till a cell reaches its maturity; on initiation the cell again divides to produce new cell and the process continues.
Cell cycle is a sequential development of cell between two cell divisions.
The cycle is genetically controlled and are programmed in every cell and are specific for each region.
Varied species has variable time length of cell cycle decided by physiological and influences pertaining to their niche.
Cell Cycle History
Discovery of microscope was a breakthrough in understanding the physiology at cellular level.
Robert Hooke was first to observe cork cells; from there microscopic observation revealed the world of microbiota.
The cell was discovered and components, functions and the significance were found. In late 19th century; it was deducted that new cell develops from the pre – existing cell by the process of cell division.
Cell division led to elucidation of cell cycle processes which was determined to be the principal process regulating growth and development of the cell and the organism.
But the DNA replication restricted to S phase of the cell cycle was not discovered until 1950 by Alma Howard and Stephane Pele.
The conservative nature of the cell cycle was identified in 1980’s; were scientists found out that the molecular processes of cell cycle is similar in all eukaryotic organisms; from experimenting various research on organisms.
Cell cycle regulators and genes “START” was discovered by Leland Hartwell.
Paul Nurse discovered CDK which runs the cell cycle based on the genetic expression.
Timothy Hunt discovered cyclin activating subunit of CDK’s. Leland Hartwell, Paul Nurse and Timothy Hunt shared Nobel Prize for Medicine in 2001.
Characteristic of Cell Cycle
o The sequential progress of cell growth and cell division to form new daughter cells are termed as cell cycle.
o The sequences are divided into 4 divergent phases falls in 2 majors they are: Interphase and Mitotic Phase.
o The stages of cell cycle is common for eukaryotic organisms.
o The cell cycle through evolution is conservative because of the similarities of the phases in all organisms.
o The cycle is well regulated by genes and protein kinases.
o Protein kinase action is same in all eukaryotic species signifies the conserved evolution and a proof that all organisms arise from a common ancestor who diverse from single celled unicellular organism to complex human beings.
o The stages of cycle are governed by time at which cells enter the cell cycle.
o Cell cycle is asynchronous in a living organism.
o The cell division is regulated and controlled by the protein kinases to maintain the structural integrity and functionality of the organism.
o As the organism grows few regions remains constant with no cell growth or maintained to a threshold level where cell death is balanced by cell division.
o In few parts the cells the growth might be rapid or enter the cycle first and other cells may enter later result in asynchronous cell division.
o Length of cell cycle varies from species to species. Few organisms have rapid replication system compared to other organism.
o For Example: Yeast completes its cell cycle in 90 minutes (about 1 and a half hours) whereas in human cell cycle lasts for 24 hours and in Drosophila it is about 8 minutes.
o Phase of cell cycle is sequential in continuously dividing cells were one phase follows the other in a constant order becomes inevitable for a proper functioning of the cell cycle are regulated by the controls of the cycle.
o In multicellular organisms, higher level of cell differentiation reduces the capacity of a cell to proceed the cycle.
o A post – mitotic differentiated cell restricts it at G1 phase or enter G0 phase a quiescent phase.
o Stem cells are an ideal example for cell cycle where it keeps on proliferating to maintain the integrity of the organism replacing the dead cells by new ones.
o Most of the cells completes the cycle whereas few other remains in the synthetic phase their lifetime without entering the dividing phase.
o Proceeding the phases of cell cycle is determined by the synthesis phase where a cell cycle sets up an upper threshold level which must be crossed to enter the division phase.
Cell Cycle Phases
The cell cycle is common for all eukaryotic organisms; travelling through 2 major phases based on the cell division: Interphase and Mitotic Phase.
Interphase consists of 3 phases Gap 1 phase, Synthesis Phase and Gap 2 Phase.
Similarly, mitosis has four phases Prophase, metaphase, anaphase, and telophase.
The development of cells through these phases are influenced and facilitated by heterodimeric protein kinases – Cyclin and Cyclin Dependent Kinases.
I. Interphase
Interphase of the cell cycle is non dividing phase where the cells undergo major synthesis and other integrating functions transducing signals and coordinate with other cells to produce an overall reaction to the stimulus.
Biosynthetic activity is more prevalent in this phase; absence of cell division marks a resting period; varies for every differentiated cells.
Certain cells might have a short resting period and certain cells such as neurons loses the power of differentiation and remains constant.
II. Gap1 Phase
The first gap phase starts just after the 2 daughter cells formed by mitosis is long and are species specific.
Intermediate to Mitosis and the synthesis phase the cell increases in size by synthesizing proteins and RNA for Synthesis Phase.
In a cell cycle, G1 phase is regulated by the external and internal factors.
External limiting factor is the availability of the nutrients in general eukaryotic organism and additional hormonal induction in plants.
Most of the organism restrict their cycle at G1 or enter G0, few cells on proper hormonal induction will reenter the cell cycle from G0 phase; specifically, plants have a specialized region called the meristem which continuously divide to produce cell growth.
The G1 phase determines the fate of the cell whether to enter cell cycle or to retain in G1 phase.
The cell must grow to an appropriate size and must synthesize proteins and RNA to enter S phase, and this is regulated by Cyclins and CDK’s.
III. Synthesis Phase
The threshold point – restriction point in mammals and START in other organisms; when crossed the cells enter the synthesis phase.
The G1 phase prepared the cell and its components to synthesis phase to get committed to cell division.
The cell division is arrested when external aids are less or unavailable.
The DNA replication doubles the genetic material, but the chromosome is not condensed and replicated material remains as chromatin.
Synthesis of genetic material is significant in this phase where cells are committed to divide and produce 2 daughter cells in most of the metazoans.
The S phase does not induce the increase in chromosomal numbers but doubles the genetic material.
Endoreduplication a landmark in plant endosperm is the different process of the metazoans permits DNA replication without mitosis results in ploidy.
In animals, Drosophila’s salivary glands and mammal’s hepatocytes exhibit endoreplication.
IV. Gap2 Phase
Synthesis phase and Mitosis phase is separated by G2 Phase; marked by absence of synthetic function.
This phase is a preparatory phase where the cell again enters the mitotic cycle for division.
Decondensed chromatin starts a preliminary condensation marks the start of mitotic phase.
Condensation inhibits the RNA synthesis gradually as the synthesis sites are being compactly wound for replication.
G2 Phase has minimal RNA synthesis which reduces or absent in Mitotic phase.
V. Mitotic Phase
The changes in above phases are minimal or not clearly visible in microscopes whereas the changes in M Phase are easily detectable.
The phase has 4 parts in which the division takes place systematically and continuously.
The cell stages are easily visible in plant parts as the specialized dividing region – MERISTEM is prevalent in roots and shoots are continuously dividing providing a mechanical support and functional integrity to the plants.
The 4 phases are: Prophase, Metaphase, Anaphase and Telophase.
Each phase has a distinctive change to be identified and Eukaryotic cells replicates in the same order in most of the organisms.
Prophase: The prophase is marked by chromosomal condensation and disintegration of cellular components and assembly of cytoskeletons for cell division. RNA synthesis is inhibited.
Metaphase: Nuclear membrane is eliminated completely chromosomes are completely condensed. The cytoskeleton – spindle fibers attach to the kinetochores. The chromosomes are aligned in the equatorial plate.
Anaphase: Chromosomal split forms daughter chromatids; travels to the opposite poles. The chromosomes are V – Shaped as they are dragged to the opposite sites.
Telophase: Microtubules disappear and chromosomes decondense to chromatin mass. Nuclear envelope starts to form. The disintegrated organelles form again.
These phases mark the Karyokinesis were the nucleus and other cell parts are newly formed.
Cytokinesis is the formation of daughter cells after mitosis; indicated by furrow which starts to differentiate two daughter cells grows gradually forming a cell plate while the organelles formed gets segregated. Cell plate represents the lamella between 2 cell walls.
Cell Cycle Regulation
The cell cycle is regulated by heterodimeric protein which was first discovered in sea urchin; the cyclic appearance named the compound CYCLIN; is the regulatory subunit activating Cyclin dependent Kinases – Catalytic subunit.
The involvement of CDK’s promotes an intricate network of mechanism triggering positive and negative response creates a loop controlling the cell cycle.
The protein kinases phosphorylate other repressors of the cell cycle traversing cell over distinct phases of development.
The cell cycle and the basic protein kinases were conserved throughout the eukaryotic cells, but the CDK types of serine – threonine special class are not conserved; varying from quantity to Plant types
The Animals has a distinctive CDK types; CDK1 – CDK7 are present in animals and CDK A – CDK E in plants.
Cyclins also falls under 5 types A, B, C, D, and H. The cyclin is transcript from most conserved part of the human genome preserving its integrity over their period.
CDK and Cyclin combines to regulate the cell cycle at each phase; characterized by specific combination of Cyclin and CDK promoting the cell to reproduce for the growth of an organism.
Distinct phases have different cyclin – CDK complex varying in animals and plants.
Monomeric CDK does not participate in regulation of cell cycle; CDK require cyclin subunit to be activated by CDK activating kinases.
These heteromeric protein can be inhibited by inhibiting proteins – Kip-related proteins by inhibitory phosphorylation.
The CDK’s and cyclins are dependent on the rate of turnover, transcription, and translational control.
Cell cycle Regulation in Plants
The CDK – CYC (cyclin) complex regulates the cell cycle. Each phase expresses specific complex for the cells to be accustomed to proceeding through cell cycle. CDK – A;1 is expressed in all phases but are activated by D type cyclins in G1 Phase to traverse the restriction point.
Transcription of CDK A – CYC D accumulates and facilitates the preparation of cell to transverse G1/S phase to commit to cell cycle.
The synthesis of CDKA – CYCD complex when increase in quantity reaches a restriction point and when it exceeds the point the cell commits to cell division.
When the point is not crossed the cell remains in G1 phase and eventually enters G0 phase indicating maturation and takes part in regular cellular activities.
In S phase CDKA – CYCA is expressed during the synthesis of DNA duplicate.
Traversing from S phase the CYCD starts expressing with the entry of G2 phase along with CYCA2 – CDKA later this followed by the expression of CDKB – CYCB which on reaching a threshold limit indicates the cells are ready for the cell division.
Again, at the end of G2 phase the positive feedback of CDKB-CYCB induces the CDKA-CYC3 to be produced all along the mitosis and prevails in G1 PHASE to increase the growth of the cell.
Cell Cycle Plant Hormones
Plant growth hormones Auxins, cytokinin and brassinosteroids has a significant role in control of the cell cycle.
Auxin and Cytokinin are essential for cell proliferation and are widely expressed in the meristematic tissues of plants promotes plant growth by cell division induces the cell cycle.
These were discovered from the mutational studies inhibiting the hormones and observing cell cycle.
Cytokinin helps in G1/S transition and G2/M transition by dephosphorylating CDK’s thereby activating the cell cycle or inhibits WEE 1 kinase by down regulation.
Abscisic acid inhibits cell cycles by promoting inhibitors such as KRP1 in certain plants.
Similarly, Jasmonic Acid inhibits DNA replication when administered during G1 phase and has minimal inhibition effects in later stages.
Ethylene promotes cell death in the developing cell cycle.
Cell Cycle Citations
- Cell cycle checkpoints. Curr Opin Cell Biol . 1994 Dec;6(6):872-6.
- Cell Cycle Control: A System of Interlinking Oscillators. Methods Mol Biol . 2016;1342:3-19.
- Cell cycle development. Dev Cell . 2004 Mar;6(3):321-7.
- Linking the Cell Cycle to Cell Fate Decisions. Trends Cell Biol . 2015 Oct;25(10):592-600.
- The cell cycle: a review. Vet Pathol . 1998 Nov;35(6):461-78.
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Phenylalanine Deaminase Test: Result, Principle, Procedure, and...
Continue ReadingPhenylalanine Deaminase Test Introduction
Biochemical tests are usually performed to detect the ability of the microorganism to distinguish their enzymatic activity and to define their characteristics.
Each of the organism acts accordingly based on their surrounding enzymatic reactions and their host cell they live upon.
These biochemical tests are performed with the help of inoculating cultures and specific chemical reagents and the indicators used.
Phenylalanine deaminase test is one such test which is used to differentiate the species belonging to the group of urea-positive gram-negative bacilli on the basis of their ability to produce phenyl.
What is Phenylalanine Deaminase Test?
Phenylalanine deaminase test is commonly known as Phenyl pyruvic acid test, which is commonly called as PPA test.
Phenylalanine deaminase test is due to ability of the organism to produce deaminase.
Phenylalanine deaminase enzyme helps in removing the amino group from the amino acid phenylalanine which thus produces and phenyl pyruvic acid and ammonia by the process of oxidative deamination of phenylalanine.
Phenyl pyruvic acid reacts along with the ferric iron and produces a visible green color.
Phenylalanine agar, commonly known as phenylalanine deaminase medium, as it contains the DL- phenylalanine and the nutrients in their medium.
Phenylalanine Deaminase Test
Phenylalanine deaminase tests is usually used to determine the ability of the organisms to produce the enzyme deaminase.
Micro organisms which are capable of producing phenylalanine deaminase.
Micro organisms that are capable of producing the enzyme phenylalanine deaminase removes the amide group from the phenylalanine thus results in release of the free molecules of ammonia.
Thus, the deamination of phenylalanine results in the formation of phenylalanine pyruvic acid with the helps of the oxidative enzymes.
Hendriksen in the year 1950 demonstrated the species of proteus which helps us in converting the amino acid phenylalanine into the acid form known as phenylpyruvicacid.
But in later times, Buttiaux et al. developed a medium of culture for detecting the formation of phenyl pyruvic acid from the members of the phenylalanine namely Proteus, Providencia and other groups such as Morganella.
This medium is modified again by Ewing et al. and further by Bynae. Bynae simplified this medium by discarding the process of protease peptone in the medium.
Usually, the micro-organisms which detects the phenylamino acid by the enzyme phenylalanine deaminase is identified by adding 4 to 5 drops of ferric chloride.
Whereas addition of ferric chloride results in the production of green colored complex between the compounds of the slant test and also in the tube which indicates the positive results.
On the other hand, the absence of green colored complex determines the negative result of the test.
Phenylalanine Deaminase Test
• The main aim of the test is to detect the ability of the organism to deaminate the phenylalanine oxidatively and to convert it into phenyl pyruvic acid.
• To differentiate the certain species of gram-negative bacilli from the Enterobacteriaceae family.
Phenylalanine Deaminase Test Reagents
Materials required for preparing a medium:
Ingredients Gram/Liter DL – Phenylalanine 2.0 Yeast extract 3.0 Sodium chloride 5.0 Disodium phosphate 1.0 Agar 12.0 Distilled water 1 liter pH 7.3 Here, yeast extract plays an important role in acting as a source of carbon and nitrogen. On the other hand, meat extract and protein hydrolysates cannot be used as the natural varying agent of the phenylalanine.
Phenylalanine Deaminase Test Procedure
Initially a loop full of inoculum is collected from an 18 to 24-hour pure culture, and it is streaked against the slanted surface with the help o a fishtail motion or by using a Phenylalanine inoculate of about 1 drop from the 24-hour brain-heart infusion medium.
The prepared medium is inoculated at a temperature of about 35 degree Celsius for about 18 to 24 hours.
After completing the process of incubation, about 4 to 5 drops of 10% ferric chloride solution is applied directly upon the slant.
Then the tube containing a slant is agitated gently and if the results are positive then the development of green color can be observed within a minimum of 1 minute and a maximum of 5 minutes.
Phenylalanine Deaminase Test Result
Positive Result: In case of positive result, the development of green color can be observed along the slant after adding few drops of ferric chloride. This reaction is usually seen within 5 minutes of addition of ferric chloride.
Negative Result: Here the negative results are determined by absence of green color in the medium after 6 to 7 minutes of addition of a ferric chloride. But negative results are determined by appearance of yellow color which is due to the presence of the ferric chloride.
Phenylalanine Deaminase Test Uses
This test is usually recommended for differentiating the species of gram-negative enteric bacilli on the basis of the ability of the micro-organism to produce the phenyl pyruvic acid by the process of oxidative deamination.
Generally, the genera like Morganella, Proteus and Providencia are differentiated by following these tests to differentiate them from Enterobacteriaceae family.
Phenylalanine Deaminase Test Citations
- Rapid identification of micro-organisms from urinary tract infections by beta-glucuronidase, phenylalanine deaminase, cytochrome oxidase and indole tests on isolation media. J Med Microbiol . 1994 Dec;41(6):389-92.
- Rapid test for urease and phenylalanine deaminase production. Appl Microbiol . 1971 Mar;21(3):545.
- Combined Medium to Determine Deoxyribonuclease Activity and Phenylalanine Deamination by Enterobacteriaceae. Appl Microbiol. 1970 Feb; 19(2): 385–386.
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