The cell cycle in eukaryotes usually comprises two distinct processes, namely, mitosis and meiosis. Mitosis occurs in somatic cells and ensures the division of a parent cell into two daughter cells that carry the same number of chromosomes as were present originally in the parent cell. Hence, the level of ploidy is maintained, or the division of chromosomes is equational, in mitosis. However, things are vastly different in meiosis. Meiosis is strictly a feature of progenitor gametic cells. This kind of cell division is crucial in the sexual reproduction of eukaryotes. At the end of meiosis, four daughter cells are formed, and each daughter cell comprises exactly half the original number of chromosomes that were present in the parental cell. That is to say, that meiosis is a reductional division. How is this process carried out? What exactly is the significance of the reduction of ploidy in the germ cells of eukaryotic organisms? This tutorial attempts to provide an answer to all such questions.
Meiosis is a specialised cell division which occurs in primordial gamete cells and essentially restores the original number of chromosomes in sexually reproducing organisms.
It's also referred to as reductional division.
Meiosis can be studied under two phases: meiosis I and meiosis II.
Meiosis I is the stage where the actual reductional division takes place, while meiosis II is analogous to the process of mitosis, and hence is also called equational division.
Four gamete cells are produced as a result of meiosis. These gametes (sperms or eggs) comprise half the amount of chromosomes that were originally present in the parent cell.
In this phase, the diploid parent cell containing homologous pairs of chromosomes divides into two daughter cells.
Meiosis I comprises of following stages −
Prophase I
Comprises 5 stages, namely, leptotene, zygotene, pachytene, diplotene and diakinesis.
Prophase I is characterised by the events of synapsis (zygotene) and crossing over and exchange of genetic segments between the non-sister chromatids of the homologous pair (pachytene).
By the end of prophase I, the long, thin threads of chromatin condense into chromosomes.
The nuclear membrane disappears.
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Metaphase I
The recombined homologous pairs eventually align at the metaphasic plate.
The arrangement of the homologous pairs at the equator is random.
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Anaphase I
This phase is characterised by the separation of the homologues of a pair from each other.
The sister chromatids of each homologue remain intact.
Telophase I
Chromosomes reach the poles on their sides.
The nuclear envelope starts to reform, surrounding the haploid set at each pole.
Cytokinesis occurs, resulting in two daughter cells.
Remember, each daughter cell now comprises a reduced number of chromosomes.
Towards the end of telophase, the reformation of chromatin occurs.
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The two daughter cells at the end of meiosis I transition into meiosis II via a stage known as interkinesis. During this phase, the nuclear membrane redevelops, the spindle apparatus breaks down and the chromosomes decondense.
Meiosis II proceeds through four stages, which are very much identical to the four stages of mitosis.
Hence, meiosis II is often called equational division, since the number of chromosomes in the daughter cells remains the same as was in the parent cells.
Meiosis II ends with the production of four daughter cells, two from each of the parental cells.
Prophase II
Recondensation of the chromosomes.
Each chromosome comprises non-identical sister chromatids (due to recombination during meiosis I).
Reformation of the bipolar meiotic spindle apparatus.
The disintegration of the nuclear membrane.
Metaphase II
The chromosomes assemble at the equatorial metaphasic plate.
Microtubules called kinetochore microtubules attach to the centromeres of each chromosome, aligning them at the centre of the cell.
Anaphase II
The sister chromatids of each chromosome at the metaphase plate separate, and each sister chromatid moves to its respective pole.
Kinetochore microtubules contract and pull the sister chromatids towards their respective poles.
Telophase II
Decondensation of the chromosomes.
Nuclear membrane reforms around the haploid set of chromosomes at each pole.
Finally,cytokinesis occurs, via the formation of a cleavage furrow, separating the two haploid nuclei into two separate cells.
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The production of genetically variable gametes is probably the most defining significance of meiotic division.Crossing over is the physical manifestation of the inheritance of variation among the offspring.
The reduction in the amount of chromosomes in the cell to half is important to ensure that the original ploidy of the organism is maintained. In humans and other diploid organisms, when the haploid gametes (n) fuse during fertilization, a diploid zygote (2n) is produced.
Meiosis is essentially the molecular basis of the law of segregation, and the law of independent assortment manifested in the formation of gametes and the random positioning of the homologous pairs of chromosomes during metaphase I.
Errors during meiosis can lead to genetic disorders.
Meiosis strictly occurs in primordial gamete cells, resulting in four daughter cells from each parental cell. The defining feature of meiosis is that it reduces the number of chromosomes in the daughter cells to half of what is present in the parent cells. This reductional division is significant in sexually reproducing diploid organisms since the gametes must be haploid to fuse and restore the diploid nature of the zygote. Meiosis is studied under two phases- meiosis I and meiosis II, each of which occur through 4 stages, namely prophase, metaphase, anaphase and telophase. Meiosis is also significant because it explains Mendel’s laws of inheritance, at the chromosomal level.
Q1. Which stage during meiosis is the molecular manifestation of the law of independent assortment?
Ans: The law of independent assortment is molecularly manifested during the metaphase of meiosis I.
Q2. At which stage of meiosis I and II are the oocytes arrested, and when is meiosis resumed?
Ans: Oocytes are arrested during diplotene of meiosis I and the process resume just before ovulation. After ovulation, a second arrest occurs at metaphase II.
Q3. What is the main difference between meiosis II and mitosis?
Ans: The main difference between mitosis and meiosis II is that the interphase of meiosis II lacks an S-phase.
Q4. In what order does meiosis II occur in the two cells obtained at the end of meiosis I?
Ans: The two daughter cells produced at the end of meiosis I synchronously undergo meiosis II, and not one after the other.
Q5. What are the genetic disorders related to improper meiotic cell division?
Ans: Nondisjunction of homologues during meiosis I or of sister chromatids during meiosis II leads to aneuploidy, such as trisomy 21 aka Down’s syndrome or trisomy 18, i.e., Edward’s syndrome, etc.
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