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Meiosis is the process in which a single cell divides twice to form four haploid daughter cells. These cells are the gametes – sperms in males and eggs in females. Unlike in mitosis, which is equal division, meiosis half the usual number of chromosomes, forming haploid cells. This reduction is crucial because when gametes fuse during fertilization, the embryo contains the correct number of chromosomes. 

The process of meiosis is divided into 2 stages. Each stage is subdivided into several phases.

Meiosis I: Prophase I Metaphase I Anaphase I Telophase I Cytokinesis I

Meiosis II: Prophase II Metaphase II Anaphase II Telophase II Cytokinesis II

Meiosis I

Mitotic cell division is equational in nature while meiosis is a reduction division. The salient features of meiotic division that make it different from mitosis are as follows:-

1. It occurs in two stages of the nuclear and cellular division as Meiosis I and Meiosis II. DNA replication occurs, however, only once.
2. It involves the pairing of homologous chromosomes and recombination between them.
3. Four haploid daughter cells are produced at the end, unlike two diploid daughter cells in mitosis.

Meiosis I separates the pair of homologous chromosomes and reduces the diploid cell to haploid. It is divided into several stages that include, prophase, metaphase, anaphase and telophase.

Prophase I

• Prophase I is longer than the mitotic prophase and is further subdivided into 5 substages.
  • Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis
• The chromosomes begin to condense and attain a compact structure during leptotene.
• At pachytene stage, crossing over of non-sister chromatids of homologous chromosomes occurs at the recombination nodules. The chromosomes remain linked at the sites of crossing over.
• Diplotene marks the dissolution of the synaptonemal complex and separation of the homologous chromosomes of the bivalents except at the sites of cross-over. The X-shaped structures formed during separation are known as chiasmata.
• Diakinesis is marked by the termination of chiasmata and assembly of the meiotic spindle to separate the homologous chromosomes. The nucleolus disappears and the nuclear envelope breaks down.

Metaphase I

The bivalents align at the equatorial plate and microtubules from the opposite poles attach to the pairs of homologous chromosomes.

Anaphase I

The two chromosomes of each bivalent separate and move to the opposite ends of the cells. The sister chromatids are attached to each other.

Telophase I 

The nuclear membrane reappears and is followed by cytokinesis. This gives rise to a dyad of cells.

Meiosis II

In Meiosis I, we have seen phase I of the meiotic cell division. Meiosis II also comprises the four stages and are relatively simple as compared to Meiosis I. Meiosis II relates the mitotic cell division.

The four stages of meiosis II are as follows:-

• Prophase II – It immediately sets off after the cytokinesis when the daughter cells are formed. The chromosomes begin to condense accompanied by the dissolution of the nuclear membrane and the disappearance of the Golgi apparatus and ER complex.
• Metaphase II – The chromosomes are connected to the centriole poles at the kinetochores of sister chromatids through the microtubules. They also get aligned at the equator to form the metaphase plate.
• Anaphase II – In this phase of meiosis II,  there is a simultaneous splitting of the centromere of each chromosome and the sister chromatids are pulled away towards the opposite poles. As the chromatids move towards the poles, the kinetochore is at the leading edge with the chromosomal arms trailing.

• Telophase II – The chromosomes dissolve again into an undifferentiated lump and a nuclear envelope develops around it. Followed by cytokinesis, telophase II marks the end of meiosis. Four haploid daughter cells are formed as a result.
• Cytokinesis: This is the division of the cytoplasm and other cell organelles into two daughter cells. In animal cells, a cleavage furrow forms and pinches the cell into two parts. In plant cells, a cell plate forms between the divided nuclei, gradually developing into a new cell wall.

Crossing Over

Crossing over is an important genetic phenomenon that occurs during the process of meiosis, specifically during prophase I. Meiosis is the cell division process that creates gametes (sperm and egg cells) with half the number of chromosomes of a normal cell. Crossing over contributes to genetic diversity by shuffling genetic material between homologous chromosomes.

Without meiosis, there would be no genetic variety, no siblings with different personalities, and ultimately, no evolution. It’s nature’s way of ensuring variety and survival. Meiosis is all about diversity.