16.1.3—Reduction division

Meiosis is a form of nuclear division in plants and animals that produces haploid cells for sexual reproduction. It starts from a diploid cell and includes one round of DNA replication followed by two nuclear divisions: meiosis I and meiosis II.

1. DNA is replicated once, so each chromosome consists of two sister chromatids joined at a centromere.
2. In meiosis I, homologous chromosomes pair and whole homologues are separated, reducing the chromosome set number; the centromeres do not divide at this division.
3. There is no further DNA replication before meiosis II. In meiosis II, centromeres divide and sister chromatids separate.
4. Cytokinesis after the two divisions produces four haploid cells. In animals these develop as gametes; in plants the products contribute to sexual reproduction.

Meiosis is a reduction division because the chromosome number changes from diploid to haploid before fertilisation. This allows two haploid gamete nuclei to fuse and restore the diploid number. Unlike mitosis, meiosis has two successive nuclear divisions and reduces chromosome number.

DNA replication happens once, not before each division. Meiosis I separates homologues; meiosis II separates sister chromatids. The detailed PMAT image-reading method and the mechanisms that generate genetic variation are covered separately.