Meiosis definition

• In sexually reproducing eukaryotes, meiosis creates four daughter cells (gametes) with half as many chromosomes as the original diploid parent cell.
• The haploid cells split to produce gametes, which combine with some other haploid cells to define sexual reproduction and provide a way to a new generation of diploid organisms through fertilisation.
• Meiosis takes place in the germ cells of creatures that reproduce sexually.
• The gonads are where germ cells are found in both plants and animals, albeit various species have varied meiotic times.

Purpose of Meiosis

• All sexually reproducing organisms require the meiotic process for the following reasons:
• Through the development of gametes, meiosis keeps the number of chromosomes in sexually reproducing organisms constant.
• Meiosis creates genetic diversity across species by causing the exchange of genes as a result of cell division. These variants serve as the evolutionary process’ starting points.

Stages/Phases of Meiosis

• Meiosis is made up of Meiosis I and Meiosis II, two cycles of cell division.
• Nuclear division (karyokinesis) and cytokinesis occur throughout each cycle of division (cytoplasmic division).
• The initial meiotic division consists of a prophase that is extended, during which the homologous chromosomes are in close proximity to one another and exchange genetic material.
• Similar to this, the first meiotic division results in the production of two haploid cells due to the reduction in the number of chromosomes.
• The heterotypic division is another name for the initial meiotic division.

Meiosis I consists of the following steps:

Interphase

• Similar to mitosis, meiosis also includes a pre-phase known as interphase.
• The interphase is characterised by the following features:
• The nuclear envelope is unaltered, and the chromosomes are present as scattered, lengthy, coiled, and hardly discernible threads of chromatin.
• The quantity of DNA doubles. The nucleolus’ size is greatly enlarged as a result of the build up of ribosomal RNA (rRNA) and ribosomal proteins there.
• An animal cell in interphase has two pairs of centrioles because the daughter pair of centrioles arises close to the current centriole.
• There is a significant shift that switches the cell’s direction from mitosis to meiosis in the G2 phase of interphase.
• At the commencement of the first meiotic division, the dividing cell’s nucleus begins to expand by taking water from the cytoplasm, and the nuclear volume almost triples.

Prophase I

The longest phase of the meiotic division is prophase I. The following substages are part of it:

Leptotene

• In the leptotene stage, the chromosomes get even more uncoiled and form a continuous thread when they also grow chromomeres, which are tiny structures that resemble beads.
• The chromosomes in the animal cell’s nucleus seem like a bouquet because they are still pointed toward centrioles at this stage.
• The Bouquet Level is another name for this stage.

Zygotene or Synaptotene

• The synapsis, or pairing of homologous chromosomes, marks the start of the zygotene stage.
• The synaptonemal complex, a protein-rich scaffolding, connects the paired homologous chromosomes.
• The synaptonemal complex aids in recombination or crossing over by stabilising the pairing of homologous chromosomes.
• The synapsis may start at one or more locations along the homologous chromosomes’ length.
• Synapsis can begin at the centromere and move towards the ends of the chromosomes (proterminal synapsis), or it can begin at the ends and move towards the centromere (procentric pairing).
• The synapsis can sometimes happen at different locations along homologous chromosomes (random pairing).

Pachytene

• At this point, it is impossible to tell the two chromosomes apart, since they have spiralled around one another.
• Each homologous chromosome breaks into two chromatids in the midst of the pachytene stage, but their shared centromere keeps them together.
• Because there are only two visible chromosomes at this stage, the chromosomes are referred to as bivalent or as a tetrad due to the presence of four visible chromatids.
• This stage is particularly important since it is at this stage that a significant genetic process known as “crossing over” occurs.
• Redistribution and reciprocal exchange of hereditary material between two homologous chromosomes occur during the crossing-over process.
• The non-sister chromatids are split at the point of crossover by the enzyme endonuclease.
• Following the breaking of chromatids, the non-sister chromatids of the homologous chromosomes exchange chromatid segments.
• The non-sister chromatid and the damaged chromatid segments are joined by another enzyme called ligase.
• Crossing over is the process of one non-sister chromatid from each homologous chromosome exchanging chromatin material with the other.

Diplotene

• The synaptonemal complex appears to be broken down, leaving the paired homologous chromosome’s chromatids physically connected at one or more specific locations known as
• Chiasmata in diplotene travel in a zip-like motion toward the ends of chromosomes.

Diakinesis

• The bivalent chromosomes are now more evenly and densely dispersed throughout the nucleus.
• The nucleolus vanishes and the nuclear envelope disintegrates at this stage.
• The chromatids are also still connected until metaphase, as the chiasmata reach the end of the chromosomes.

Metaphase I

• The attachment of spindle fibres to chromosomes and the equatorial alignment of the chromosomes make up metaphase I.
• The spindle fibres are joined with the homologous chromosomes’ centromeres, which are pointed in opposing directions, during metaphase I.

Anaphase I

• At anaphase, I, homologous chromosomes are split from one another, and as a result of the shortening of chromosomal fibres or microtubules, each homologous chromosome moves toward the cell’s opposite pole with its two chromatids and undivided centromere.
• The two chromatids of a chromosome are not genetically similar because one of the chromatids changed its counterpart during chiasma formation.

Telophase I

• The migration of a haploid pair of chromosomes at each pole marks the beginning of telophase I.
• The chromosomes become uncoiled when the nuclear membrane is produced around them. Two daughter nuclei are created as a result of the nucleolus’ reappearance.

Cytokinesis I

In plants, cytokinesis results in the generation of two daughter cells, while in animals, it happens when the cell membrane constricts, giving rise to two daughter cells.

Meiosis II

• The haploid cell splits mitotically in the second stage of meiosis to produce four haploid cells. The homotypic division is another name for this division.
• In contrast to the first meiotic division, the sharing of genetic material is not included in this category or a decrease in the number of chromosomes.

Following are the stages of meiosis II:

Prophase II

• Each centriole splits during prophase II, producing two pairs of centrioles.
• The nucleolus vanishes when the centrioles migrate in the direction of the nuclear membrane and the opposite poles.

Metaphase II

• Through the spindle fibres, the chromosomes are positioned on the cell’s equator during metaphase II.
• As a result, each chromosome creates two daughter chromosomes as a result of the centromere dividing.
• Each chromosome’s centromere receives a spindle apparatus attachment.

Anaphase II

The shortening of chromosomal microtubules and the stretching of the spindle’s interzonal microtubules cause the daughter chromosomes to migrate in opposing directions.

Telephase II

• The chromatids, which are now referred to as chromosomes, move to the opposing poles.
• The nucleolus reemerges as a result of the production of ribosomal RNA, and the endoplasmic reticulum forms the nuclear envelope surrounding the chromosomes.

Cytokinesis II

Each of the four daughter cells is produced by cytokinesis, which is the same as cytokinesis I and results in the division of cytoplasm.

Applications of Meiosis

Several lab-based technologies, some of which are listed below, employ meiotic similarity to mitosis:

Tissue culture

Meiosis is used in biotechnology to give cells a gametic state, much like mitosis is.

Meiosis frequently follows mitosis to provide a variety that helps with research on evolutionary processes.

In-vitro gamete formation

The embryonic stem cells are differentiated into germ-like cells by the meiotic division in a variety of infertility problems caused by gamete failure.

Meiosis is used to create these gametes in-vitro, and they are then injected into people with certain illnesses.

References

• Verma PS and Agarwal VK (3005). Cell Biology, Genetics, Molecular Biology, Evolution and Ecology. Multicolored Edition.
• Rastogi SC (2006). Cell and Molecular Biology. Second Edition. New Age International.
• Eguizabal C, Montserrat N, Vassena R, et al. Complete meiosis from human induced pluripotent stem cells. Stem Cells. 2011;29(8):1186‐1195. DOI:10.1002/stem.672
• Ronchi VN (1995). Eguizabal C, Montserrat N, Vassena R, et al. Complete meiosis from human induced pluripotent stem cells. Stem Cells. 2011;29(8):1186‐1195. DOI:10.1002/stem.672.

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