Meiosis

Meiosis, unlike mitosis, creates daughter cells that have half as many chromosomes as the parent cell (Figure 1).

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the preceding phases mitosis. During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids together until anaphase II.

Figure 1 Meiosis proceeds in two steps. During meiosis I two haploid cells are formed each containing one set of homologous chromosomes. During meiosis II the sister chromatids of these homologous pairs are separated, producing four gametes.

The function of meiosis:

• Reduce the number of chromosomes from diploid to haploid

• Ensure each daughter cells has a complete set of chromosomes

• Generate genetic diversity among the daughter cells

Meiosis consists of two nuclear divisions, meiosis I and meiosis II, that reduce the number of chromosomes to the haploid number, in preparation for sexual reproduction. In meiosis I, two unique events happen:

• Synapsis and crossing over. During prophase I, the homolog pairs up and is held by synapsis. Crossing over also happens during prophase I.

• Homologous pairs at the metaphase plate. During metaphase I, homologous chromosomes come together to pair along their entire length. This pairing doesn’t occur in mitosis.

• Separation of homolog. During anaphase I, the homologous chromosome pairs separate, but the individual chromosome, each consisting of two sister chromatids remain intact.

In meiosis II, the sister chromatids are separated.