Cell 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.