Meiosis

Meiosis is a special kind of cell division that halves the chromosome number, producing haploid cells from a diploid parent. Its role becomes clear when we consider sexual reproduction, which involves the fusion of two gametes, each carrying a complete haploid set of chromosomes.

Gametes are not made directly from any cell; they arise from specialised diploid cells. If these cells simply divided by mitosis, the gametes would remain diploid, and fusion would keep doubling the chromosome number generation after generation. To prevent this, the specialised cells undergo meiosis — a division that cuts the chromosome number in half and yields haploid daughter cells.

In this way, meiosis ensures that a haploid phase appears in the life cycle of sexually reproducing organisms, while fertilisation — the fusion of two gametes — later restores the diploid phase. Meiosis is encountered during gametogenesis, the formation of gametes, in both plants and animals, and it is this process that gives rise to the haploid gametes.

The key features of meiosis are:

• It consists of two sequential rounds of nuclear and cell division, called meiosis I and meiosis II, but only a single round of DNA replication takes place before both.
• Meiosis I begins after the parental chromosomes have already replicated during the S phase, producing identical sister chromatids.
• Meiosis involves the pairing of homologous chromosomes and recombination (exchange of segments) between the non-sister chromatids of these homologous chromosomes — the source of new genetic combinations. …