Animation 17.7 Meiosis
Textbook Reference: Meiosis and Fertilization, p. 679
Meiosis follows a round of DNA synthesis and consists of two nuclear divisions, called meiosis I and meiosis II, that divide the chromosome number by half in the resulting daughter cells. In prophase I of meiosis I, chromosomes begin to condense and homologous chromosomes form pairs.
At this stage, it becomes clear that the DNA synthesis in the preceding interphase resulted in two sister chromatids per chromosome.
A chromosome pair consists of four chromatids, called a bivalent. In a bivalent, recombination takes place between homologous chromosomes. A recombination site is visible and is called a chiasma (or plural, chiasmata).
At metaphase I, the bivalent chromosomes align on the spindle.
Anaphase I is initiated by disruption of chiasmata as homologous chromosomes separate. Sister chromatids remain associated at their centromeres.
At completion of meiosis I each daughter cell has therefore acquired one member of each homologous pair, consisting of two sister chromatids.
Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. At metaphase II, the chromosomes align on the spindle.
The link between the centromeres of sister chromatids is broken at anaphase II, and sister chromatids segregate to opposite poles. Cytokinesis then follows, giving rise to haploid daughter cells.