Meiosis

What is meiosis?

Meiosis is a type of cell division that produces four haploid daughter cells from one diploid parent cell. It involves two divisions — meiosis I and meiosis II — but the DNA is copied only once beforehand. Because of this, the chromosome number is halved: from diploid (2n) to haploid (n).

Why halving matters

Human body cells are diploid (46 chromosomes). If gametes were also diploid, a fertilised egg would have 92 chromosomes and the number would double every generation. Meiosis prevents this by making gametes with 23 chromosomes each, so fertilisation restores the diploid number of 46.

Meiosis creates variation

Meiosis introduces genetic variation among the daughter cells in two ways:

• Crossing over: matching chromosomes exchange segments, mixing the genes.
• Independent assortment: chromosome pairs line up and separate randomly, so gametes receive different combinations.

As a result, the four cells produced are genetically different from one another and from the parent — unlike the identical cells made by mitosis.

Meiosis I and meiosis II

The two divisions have different jobs. In meiosis I, the matching pairs of chromosomes (homologous chromosomes) separate, so the chromosome number is halved from diploid to haploid; this is where crossing over happens. In meiosis II, which is similar to mitosis, the sister chromatids of each chromosome separate. The result of both divisions together is four haploid cells. Because the DNA was copied only once at the start but the cell divided twice, the chromosome number ends up halved — exactly what is needed to make gametes.