What is Meiosis?

In humans, diploid germ cells in the testes and ovaries produce haploid sperm and egg cells through meiosis. Meiosis consists of two successive cell divisions. The first division is a reductional division, in which the chromosome number is reduced from diploid to haploid. The second division is an equational division, in which the chromosome number remains unchanged, producing four haploid cells.

The first division is meiosis I, which begins with diploid cells containing replicated homologous chromosomes. These are corresponding maternal and paternal chromosomes that carry the same genes. Each chromosome appears X-shaped because it contains two identical sister chromatids joined at the centromere.

During this stage, maternally and paternally inherited chromosomes pair and exchange genetic material through crossing over, creating genetic diversity.

After the nuclear membrane disintegrates, the chromosomes move into the cytoplasm. Spindle fibers, microtubule-based structures, attach to the chromosomes, separate them, and distribute them randomly and evenly between opposite ends of the cell before division.

The result is two genetically unique haploid cells.

Importantly, although each X in these newly formed cells is called a chromosome, each chromosome still consists of two attached copies, called sister chromatids.

During the second division, meiosis II, a similar arrangement of microtubules breaks apart the sister chromatids.

This produces haploid cells in which the chromosomes are no longer replicated and appear I-shaped when condensed.

Depending on whether meiosis happens in the testes or ovaries, this process produces either four sperm cells or one mature egg, along with three polar bodies, which are small cells that usually degenerate later.

When a sperm fertilizes an egg, it produces a genetically unique diploid individual that is genetically distinct from its parents because of meiotic events.