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.

Visual depiction of meiosis: interphase cell with 4 single chromosomes. Prophase 1: condensed chromosomes align in nucleus. Meiosis 1 phases: prometaphase 1 - nucleus dissolves, spindles attach to duplicated chromosomes; metaphase 1 - duplicated chromosomes pair up; anaphase 1 - sister chromatids move to poles; telophase 1 - spindles dissolve, cell pinches. Cytokinesis splits cell, forming 2 with 1 set of chromatids. Meiosis 2: prophase 2 - nucleus surrounds chromatids; prometaphase 2 - spindles attach; metaphase 2 - chromatids line up; anaphase 2 - chromatids split, move to poles; telophase 2 - spindles dissolve, cell pinches. Cytokinesis separates halves. Result: 4 haploid cells.

Figure 1 Meiosis proceeds in two steps. During meiosis I 1 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 I1, two unique events happen:

  • Synapsis and crossing over. During prophase I1, the homolog pairs up and is held by synapsis. Crossing over also happens during prophase I1.

  • Homologous pairs at the metaphase plate. During metaphase I1, homologous chromosomes come together to pair along their entire length. This pairing doesn’t occur in mitosis.

  • Separation of homolog. During anaphase I1, 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.