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Meiosis: Cell division – Definition, Physiology, and...
Continue ReadingMeiosis: Cell division - Definition, Physiology, and Examples
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Meiosis: Definition, Stages, and Examples
Continue ReadingMeiosis Definition
Meiosis is defined as the process of cell division that results in the formation of a haploid “daughter” cell with the same haploid chromosomal number as the diploid “parent” (“original”) cell. After meiosis, the haploid cell would have just one portion of the parent cell’s multiple homologous chromosomal pairs.
What is Meiosis?
Meiosis is important because it decreases the number of chromosomes to half and allows for genetic diversity through genetic recombination and independent assortment.
Meiosis creates four haploid cells that may grow into potential gametes, resulting in a new individual with the complete number of genes when fertilisation occurs, preserving chromosomal number integrity over generations while increasing genetic variety and variability in population forms.
Meiosis comes from the Greek term meioun, which means “to diminish” (less).
The generation of gametes (egg cells or sperm cells) or spores is the primary purpose of meiotic division. The meiosis process occurs in the human body to reduce the number of chromosomes in a normal cell (46 chromosomes) to 23 chromosomes in eggs and sperm. As a result, the number of chromosomes in meiosis is cut in half.
As a result, when the two haploid cells merge during fertilisation, the number of chromosomes in the generated cell is restored as somatic cells (each with 46 chromosomes).
Meiosis is split into two stages: meiosis 1 and meiosis 2. Each component contains four stages (prophase, metaphase, anaphase, and telophase), which are identical to the four phases of mitosis.
The most complex portion of meiosis (i.e., meiosis I) is the initial part of the meiotic division. Because it comprises five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis, prophase I takes roughly half the time it takes for meiosis. The chromosomes’ activity and structure change at each step, providing insight into prophase I’s intricacy.
Three major characteristics distinguish meiosis from mitosis:
1. Reverse recombination occurs during meiosis I. (also known as chiasma development or crossing across)
2. Meiosis I is characterised by the pairing of homologous chromosomes.
3. In Meiosis I, the cohesive sister chromatids are released in a two-step procedure.
On the mitotic spindle, these characteristics enable homologous segregation. The two sister homologous chromosomes separate during meiosis II. For the diverse events occurring in each meiosis stage, these varied chromosomal behaviours are detailed below. It’s also worth noting that these events are intertwined.
This indicates that distinct processes during chromosomal pairing, including reciprocal recombination, crossing-over, and chiasma formation, are linked. Hence, the only effective recombination mechanism during meiosis I prophase will be the one that provides proper homologous chromosome segregation at meiosis I.
The initial number of chromosomes is halved after the two subsequent chromosomal divisions. Meiosis I begins when the S phase is completed and the replication of the parent chromosome produces identical chromatids.
The chromosomes begin to couple together and eventually partition into two distinct cells. The chromatids, on the other hand, stay intact, and at the conclusion of meiosis I, each of the newly produced daughter cells will have one of the homologous chromosomes with two chromatids.
Meiosis II occurs after Meiosis I. After that, the two chromatids will split and form two daughter cells. As a result, four daughter haploid cells are formed at the end of meiosis II, each carrying one copy of each chromosome.
The pairing of homologous chromosomes after DNA replication not only allows for the segregation of meiotic chromosomes but also aids in the recombination of maternal and paternal chromosomes. This chromosomal pairing takes place during the prophase of meiosis I.
The first meiotic division (or meiosis I) and the second meiotic division (or meiosis II) are the two nuclear divisions that occur during meiosis (or meiosis II). The 4 key phases of meiosis are prophase, metaphase, anaphase, and telophase. They are also classified as I or II depending on whether they occur in meiosis I or II.
Meiosis Function
Consider this: if gametes (eggs and sperm) were generated only by mitotic division, rather than meiosis, the gametes would have the same number of chromosomes as diploid somatic cells. As a result, when the gametes unite during fertilisation, the resultant zygote will have four homologous chromosomal sets and will be tetraploid.
This “doubled chromosome content” situation will be passed down to future generations, resulting in chromosomal abnormalities. Throughout generations, the chromosomal number has fragmented and unkept. This is a chromosomal anomaly, to be sure.
Gametes are generated by the process of meiosis to keep the number of chromosomes constant in each generation. During the production of gametes, meiotic cell division reduces the number of chromosomes to haploid.
One cycle of chromosomal DNA replication precedes two rounds of nuclear division in meiosis. As a result of the parent cell’s meiotic division, four daughter nuclei (each of which is present in a new daughter cell) are generated.
Only a haploid number of chromosomes are found in each daughter cell nucleus. Gametes haploid cells are formed in two rounds: Meiosis I and II, with just one cycle of DNA replication (at the S phase of interphase).
Meiosis vs Mitosis
The two primary types of cell division are meiosis and mitosis. The distinctions between them are in mitosis only one nuclear division takes place and in meiosis two divisions. Meiosis produces haploid cell and they are sex cells, whereas in mitosis diploid cell are formed and are the somatic cells.
The end-product of mitosis is two daughter cells and in mitosis four daughter cells. Asexual reproduction takes place in mitosis and sexual reproduction in meiosis, thus crossing over takes place whereas in mitosis no crossing over is seen.
Phases of Meiosis
Meiosis is the process through which a parent diploid cell divides into four haploid cells. There really are two main phases of meiosis: meiosis I and meiosis II. Meiosis 1 is the first stage of meiotic division, often known as the reduction division of meiosis. This is due to the fact that the number of chromosomes is cut in half at this stage, resulting in the creation of haploid chromosomes.
In a process or occurrence known as a synapse, each pair of chromosomes comes close together to exchange a portion of their genetic material. This happens in the early phases of meiosis 1, especially during prophase I.
The homologous pairs of chromosomes come very close together and bond closely to each other during prophase 1 of meiosis I, and they virtually behave as one single unit. This unit is known as a bivalent or tetrad (indicating that each chromosome consists of two sister chromatids, so the sum of bivalent is four chromatids).
After aligning at the spindle equator, the bivalent separates into two pieces, allowing each chromosome to travel to the spindle pole on the opposing side. As a result, following meiosis I, each newly formed daughter nucleus is haploid, as it only carries one bivalent chromosome.
Meiosis II, the second stage of the meiosis cell cycle, is similar to mitosis in that it results in the formation of two daughter cells when the two chromatids separate.
As a result, meiosis I is the stage during which the meiosis cycle’s distinctive activities take place. Nonetheless, each step of the meiotic division, such as prophase, metaphase, anaphase, and telophase, is split in a manner that mimics the mitotic division.
The prophase of the first meiotic division, on the other hand, is far more complex and time-consuming than the prophase of mitosis. The prophase of the second meiotic division, on the other hand, is simpler and shorter.
Meiosis in a Nutshell
A cell’s chromosomes are replicated before it enters meiosis (during the interphase). The chromosomes condense along the nucleus’s centre during meiosis I and pair with their homologues during crossing over. The chromosomal pairs then split and travel to opposing ends of the cell.
For the first time, the cell splits into two cells. Both cells will go through meiosis II, when they will divide into two cells, each carrying one of each detached chromosome’s sister strands (chromatids), resulting in four genetically distinct haploid cells.
Meiosis Satges
Meiosis I occurs after interphase, when the chromosomes duplicate during the S phase of the cell cycle. During the early stages of prophase, I, the chromosomes condense. The cell’s two centrosomes go to the cell’s two opposing poles to prepare it for nuclear division.
Pairs of chromatids make up homologous chromosomes. After pairing, these chromosomes form bivalents in order to align at the spindle equator during metaphase I.
During anaphase, homologous chromosomes are separated from one another, but the two sister chromatids remain connected.
i. Prophase I
The most difficult phase of meiosis I is prophase I, which is split into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
The chromatin fibres condense into thread-like fibres at the Leptotene stage, resembling the established structure at the onset of mitosis.
The fibres are more condensed at the zygotene stage, allowing them to be recognised as distinct chromosomes. The bivalents arise when the pairs of chromosomes get firmly linked together as a result of synapsis.
The development of bivalent is crucial in the process of crossing over, which involves the exchange of DNA segments holding genetic material between two nearby chromosomes. During the pachytene stage, this procedure occurs. For gene recombination, the matching regions of chromosomes exchange genetic information.
During the pachytene stage, chromosomes are compressed to roughly a fifth of their original length. The two chromosomes of each bivalent split from one another at the centrosome during the diplotene stage. Chiasmata, which are links found when two homologous chromosomes swap DNA segments, keep the two chromosomes together.
Diplotene is characterised by the resumption of transcription, the de-condensation of chromosomes, and the temporary halt of meiosis. When the chromosomes are re-condensed to their maximal level of compaction at the start of prophase I’s last step, diakinesis, the centrosomes move even faster.
Only the chiasmata keep the chromosomes together. Spindles develop, nucleoli vanish, and the nuclear envelope vanishes at this point. Meiosis prophase 1 ends and meiosis metaphase 1 begins when the meiotic spindle begins to develop and the nucleoli begin to disintegrate.
ii. Metaphase I
After attaching to the microtubules with their kinetochores, the bivalents migrate to the equator of the spindle during this phase. This bivalent migration to the cell’s equator generally occurs only during meiosis I and not during mitotic division.
Each bivalent includes four chromatids, which means each bivalent also contains four kinetochores. These kinetochores appear to be in close proximity to one another, as though they are a single unit facing the same cell pole.
Each kinetochore can be attached to the microtubules of the spindle pole on the opposite side using this configuration. This arrangement is the initial step in preparing the chromosomes for separation during the anaphase that follows.
The paternal and maternal chromosomes are aligned on one pole of the cell at this time, and each newly created daughter cell will get a combination of paternal and maternal chromosomes during their migration to the opposing poles during anaphase.
iii. Anaphase I
The movement of homologous chromosomes to the spindle poles with the help of their kinetochore is the first step in anaphase. One of the key distinctions between meiosis and mitosis is this stage.
During mitosis, the sister chromatids are pushed to opposing poles, causing them to split. The two sister chromatids stay connected during meiosis, and following separation, the homologous chromosomes migrate toward the spindle poles.
As a result, by the end of meiotic anaphase I, each spindle pole contains a haploid number of chromosomes. During mitosis, chromatid separation is accomplished by cleaving the two sister chromatids using an active enzyme called separase.
To counteract separase’s activity during meiosis, the cell generates shugoshin, a protein that inhibits chromatid separation by shielding the centrosomal region of the chromosome where the cleavage occurs.
iv. Telophase I
Telophase 1 is the last phase of meiosis I, and it is marked by the movement of chromosomes to the spindle poles. Before cytokinesis, a nuclear membrane might have been created around chromosomes, resulting in two daughter cells with haploid sets of chromosomes. After the start of meiosis II, the chromosomes usually condense.
Results of Meiosis I
By the completion of meiosis, I, cytokinesis has aided in the development of two haploid nuclei cells. Each haploid cell’s chromosomes will be made up of two chromatids joined at the centromere.
Meiosis II Stages
Interphase meiosis occurs between the conclusion of meiosis I and the start of meiosis II. This stage is not connected with DNA replication since each chromosome already has two chromatids that have previously been replicated by the DNA synthesis process before the start of meiosis I.
In a nutshell, DNA is duplicated once before meiosis begins. Meiosis II, also known as second mitotic division, serves a similar goal as mitosis in that two new chromatids are orientated in two new daughter cells. As a result, the second meiotic division is also known as the meiotic division of separation.
i. Prophase II
Prophase II is the stage that occurs after meiosis I or interkinesis, and it is marked by the breakdown of the nuclear envelope and nucleolus, as well as the thickness and shortening of the chromatids, and centrosome replication and migration to the polar side. Prophase II is less complicated and shorter than prophase I, and it resembles the mitotic prophase in appearance.
Prophase II, on the other hand, differs from prophase I in that chromosomal crossing occurs only during prophase I and not during prophase II. At the completion of prophase II, metaphase II begins.
ii. Metaphase II
Metaphase II of meiotic division is identical to metaphase II of mitotic division, except that metaphase II has half the number of chromosomes and is distinguished by chromosomal alignment in the cell’s centre.
iii. Anaphase II
It is the stage after metaphase II, during which the sister chromatids split and migrate towards the cell poles. Anaphase II is similar to mitotic anaphase in that both involve chromatid separation. The shortening of the kinetochore causes sister chromatids to migrate to the cell’s two ends.
iv. Telophase II
Telophase II is the final stage of meiosis, in which four haploid cells are generated from the two cells produced during meiosis I. The newly formed cells’ nuclear membranes are fully established, and the cells are entirely separated at the conclusion of this phase.
However, sperm in humans and other animals are not completely functional at the conclusion of telophase II because they require the development of flagella to operate correctly.
Results of Meiosis II
After telophase II and cytokinesis, four haploid cells are formed, each of which has just one of the two homologous pairs of chromosomes. The genetic information from the maternal and paternal chromosomes is mixed in the haploid cells formed. These cells have a role in both the genetic variety of individuals within the same species and the evolution of animals.
Meiosis Examples
Meiosis is found in the life cycles of many creatures, including fungi, plants, algae, animals, and humans.
Meiosis can generate spores or gametes depending on the species, with gametes (sperm cells and egg cells) being produced in humans and other animals, while spores are produced in plants and algae.
Meiosis in Humans and Other Animals
In humans and other animals, meiosis generates haploid gametes. It is an important aspect of gametogenesis. Gametogenesis is the biological process of producing gametes, as the name suggests.
There are two types of gametogenesis in humans and other animals: spermatogenesis (the creation of male gametes, such as sperm cells) and oogenesis (the formation of female gametes, such as eggs) (formation of the female gamete, i.e., ovum or egg cell).
A diploid oocyte produces four haploid gamete cells during oogenesis. Only one cell survives to become an egg, while the other three become polar bodies.
This effect is caused by the oocyte’s uneven division during meiosis, in which one of the produced cells obtains the majority of the parent cell’s cytoplasm while the other generated cells degenerate, resulting in an increase in the concentration of nutrients in the formed egg. During prophase I of meiosis, the egg cell develops the majority of its specialised activities.
After meiosis and post-meiotic processes, such as spermiogenesis, when the sperm cell grows by obtaining a functioning flagellum and discarding much of its cytoplasm to form a compacted head, the sperm gets its specialised characteristics in order to develop into a functional gamete.
Meiosis is a process that happens throughout an organism’s reproductive period. However, in humans, meiotic division happens at various times. For example, in males, it begins during puberty and continues throughout their lives.
Females’ main oocytes will be stopped at prophase I by the time they reach adolescence, and they will proceed through the next phases of meiosis. However, each primary oocyte will cease at metaphase II of meiosis II when it develops into a secondary oocyte at ovulation.
Meiosis will only continue and finish during conception. Meiosis will stop if the secondary oocyte is not fertilised, and the arrested secondary oocyte will dissolve. Menstruation will start soon.
Meiosis in Plants and Algae
Plants and algae are multicellular creatures that produce both haploid and diploid cells throughout their lives. The haploid spores are generated by meiosis in this phenomenon known as alternation of generations. In plants and algae, this is also known as sporic meiosis.
The produced spores germinate and proceed through mitotic division, resulting in a haploid plant or algae. Because gametes are generated by mitotic division of already existing haploid cells, the haploid form is referred to as a gametophyte.
During fertilisation, the gametes unite to generate the diploid type of cells. Meiosis creates the spores from the diploid form. As a result, the diploid form is referred to as the sporophyte.
Meiosis in Fungi
In their life cycle, fungi have both asexual and sexual stages. The mycelium, for instance, may go through both sexual and asexual phases.
When haploid mycelia reach the sexual phase, they undergo plasmogamy (the union of two protoplasts) and karyogamy (the fusion of two protoplasts) (the fusion of two haploid nuclei). The development of the diploid zygote occurs after karyogamy.
The zygote develops into a stalked sporangium, which via meiosis produces haploid spores. Meiospores are the spores generated by meiosis, as opposed to mitospores, which are formed by mitosis. The sporangium will produce haploid spores (reproductive cells), each of which will germinate into a new mycelium.
Thus, in fungi, meiosis is the third step in the sexual phase’s sequential phases, which begin with plasmogamy and end with karyogamy. Meiosis is necessary for the fungus to return to its haploid stage.
Errors in Meiosis
Meiosis is prone to mistakes, which might have an impact on a person’s capacity to reproduce. Human longevity is severely harmed by abnormal meiosis. Infertility and the production of genetically unbalanced gametes can both be caused by errors in the meiosis stages.
Meiotic errors are the primary cause of congenital malformations as well as mental abnormalities in new born infants caused by genetic damage.
In more than 30% of human oocyte pachytene, errors in chromosomal pairing and recombination are present, resulting in a condition known as asynapsis, in which homologous chromosome pairing fails.
Failure of chromosomal pairing in yeast can cause cell death by triggering the cell’s checkpoints. A similar phenomenon may be seen in human germ cells. As a result of the rise in oocytes with chromosomal pairing faults, the number of germ cells will be depleted, resulting in early menopause in women.
Infertility results from mistakes in the phases of meiosis of spermatocyte development, as the quantity of functional sperm generated decreases.
Because a man generates around 300-400 million sperm each day, but a woman produces about 300-400 oocytes over her lifetime, depletion in the number of germ cells is more substantial in females than in males.
When chromosomal pairs fail to cross over properly during metaphase I, the unpaired chromosomes segregate randomly, increasing the chance of producing aneuploid gametes with an unbalanced number of chromosome copies. Furthermore, due to unsuccessful crossing-over, spermatocytes may be destroyed by apoptosis or necrosis.
Biological Importance of Meiosis
Because the balance between the number of chromosomes that are doubled during fertilisation and the halving of chromosomes during gamete production is maintained, meiosis and mitosis are two critical phases of the cell cycle for any organ that reproduces sexually.
A sexually reproducing organism’s cell cycle is divided into two distinct phases: haploid and diploid.
The haploid phase of meiosis begins with gamete creation and concludes with the development of a zygote during fertilisation, whereas the diploid phase begins with the formation of a zygote by the fusing of two gametes and terminates with meiotic cell division during gamete formation.
To complete the life cycle of sexually reproduced creatures such as humans and animals, meiotic division creates four haploid cells from one diploid cell.
The chromosomal DNA doubles in the parent diploid cell before meiosis, and four haploid nuclei are produced as a result of two subsequent diploid nucleus divisions.
Meiosis is physiologically significant since it is responsible for sexually reproduced organisms’ genetic variety, when the chromatids of two homologous chromosomes synapse and exchange portions of their genetic content during prophase I.
Meiosis is important because it decreases the number of chromosomes by half and allows for genetic diversity through genetic recombination and independent assortment.
Meiosis creates four haploid cells that may grow into potential gametes when fertilisation occurs, resulting in a new individual with the complete number of genes when fertilisation occurs, preserving chromosomal number integrity over generations while increasing genetic variety and variability in population forms.
Meiosis Citations
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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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Meiosis: Phases, Diagram, Stage, and Checkpoints
Continue ReadingWhat is Meiosis?
o Meiosis is double nuclear division which produces four haploid gametes (also called germ cells).
o In humans, only the spermatogonium and the oogonium (both diploid) undergo meiosis.
o All other cells are somatic and undergo mitosis.
o The gametes are haploid.
o After replication occurs in S phase of interphase, the cell is called a primary spermatocyte or primary oocyte (both diploid).
o In the human female, replication takes place before birth, and the life cycle of all germ cells are arrested at the primary oocyte stage until puberty.
o Arrested in prophase 1 Just before ovulation, a primary oocyte undergoes the first meiotic division to become a secondary oocyte (haploid).
o The secondary oocyte is released upon ovulation, and the penetration of the secondary oocyte by the sperm stimulates anaphase II of the second meiotic division in the oocyte.
o Meiosis is two rounds of division called meiosis I and meiosis II.
o Meiosis I proceeds similarly to mitosis with the following differences.
Diagram of Meiosis Phases
Stages of Meiosis
Prophase I
In prophase I homologous chromosomes line up along side each other, matching there genes exactly.
o At this time, they may exchange sequences of DNA nucleotides in a process called crossing over (synapsis).
o Genetic recombination in eukaryotes occurs during crossing over.
o You can also have Double crossovers:
Scenario 1: results in no genetic recombination. The chromatids involved in this double crossover exchange alleles at first, but then it exchanges them back, resulting in no net recombination. This is called the 2-strand double crossover. Results in 0/4 recombinants.
o Scenario 2: results in genetic recombination. The chromatids exchange alleles during a crossover. Then, one of the crossover chromatid exchanges with a different chromatid. This is called the 3-strand double crossover. Results in 2/4 recombinants.
o Scenario 3: results in genetic recombination. The chromatids exchange, then 2 totally different chromatids on the same chromosome exchange. This is called the 4-strand double crossover. Results in 4/4 recombinants.
o Since each duplicated chromosome in prophase I appear as an ‘x’, the side by side homologues exhibit a total of four chromatids, and are called tetrads.
o If crossing over does occur, the two chromosomes are “zipped” along each other where nucleotides are exchanged, and form what is called the synaptonemal complex.
o A chiasma (plural: chiasmata) is thought to be the point where two homologous non-sister chromatids exchange genetic material during chromosomal crossover during meiosis.
o Sister chromatids also form chiasmata between each other, but because their genetic material is identical, it does not cause any change in the resulting daughter cells.
o Genes located close together on a chromosome are more likely to cross over together, and are said to be linked!!!
Metaphase I
o In metaphase I the homologues remained attached, and move to the metaphase plate.
o Rather than single chromosomes aligned along the plate as in mitosis, tetrads align in meiosis.
Anaphase I
o Anaphase I separates the homologues from their partners, the centromeres stay together (which is different from anaphase in mitosis).
Telophase I
o In telophase I, a nuclear membrane may or may not reform, and cytokinesis may or may not occur. In humans the nuclear membrane does reform and cytokinesis does occur.
o If cytokinesis occurs, the new cells are haploid with 23 replicated chromosomes, and are called secondary spermatocytes and secondary oocytes.
o In the case of a female, one of the oocytes, called the first polar body, is much smaller and degenerates (it may or may not go through meiosis II).
o This occurs in order to conserve cytoplasm, which is contributed only by the ovum.
"These four phases together are called meiosis I. Meiosis I is reduction division"
o Meiosis II proceeds with prophase II, metaphase II, anaphase II, and telophase II.
o The final products are haploid gametes each with 23 chromosomes.
o In the case of the spermatocytes, four sperm cells are formed. In the case of the oocyte, a single ovum is formed.
o In the female, telophase II produces one gamete and a second polar body.
o If during anaphase I or II the centromere of any chromosome doesn’t split, this is called nondisjunction (it can also happen in mitosis but the ramifications are less severe).
o As a result of primary nondisjunction (nondisjunction in anaphase I), one of the cells with end up with two extra chromatids
o Complete extra chromosome) and the other will be missing a chromosome.
o If nondisjuction occurs in anaphase II that will result in one cell having an extra chromatid and one lacking a chromatid.
o The number of different possible gametes that can be formed by diploid organisms as a result of independent assortment of chromosomes during meiosis can be calculated by using the formula 2^n where n is the number of heterozygous genes.
o Ex. AaBbCc ⇒ this can produce 2^3 number of different haploid cells.
o Parthenogenesis (means the growth and development of an embryo or seed without fertilization by a male.
o Parthenogenesis occurs naturally in some lower plants, invertebrates (e.g. water fleas, aphids) and some vertebrates (e.g. lizards, salamanders, some fish, and even turkeys).
o Parthenogenetic populations are typically all-female.
o As with all types of asexual reproduction, there are both costs and benefits associated with parthenogenesis.
o Spermatogonium (diploid) ⇒ primary spermatocyte (diploid) ⇒ 2ndary spermatocyte (haploid) ⇒ spermatid (haploid) ⇒ spermatozoa (haploid)
o Oogonium (diploid) ⇒ primary oocyte (diploid) ⇒ secondary oocyte (haploid) ⇒ zygote (diploid)
o A Barr body is a permanently inactivated X chromosome that forms a dense stainable nuclear mass.
o A normal female with XX inactivates one of her X’s while expressing the other.
o She therefore has 1 Barr body.
o A normal male, who is XY, does not inactivate his only X chromosome, and therefore has no Barr bodies.
o The rule is that the number of X chromosomes is always 1 MORE than the number of Barr bodies.
o Ex. A person with 2 Barr bodies has to have 3 X chromosomes.
Meiosis Citations
- Elevated Mutagenicity in Meiosis and Its Mechanism. Bioessays . 2019 Apr;41(4):e1800235.
- 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.
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