Meiosis

Meiosis is the process by which a diploid (2n) cell forms four gametes. Meiosis differs from mitosis as meiosis I involves chromosomal content being recombinantly halved.

Overview of Meiosis

Meiosis Overview

Meiosis is involved in producing genetic variation within a species and is part of the process of sexual reproduction. Two stages occur in meiosis, each split down into several fundamental processes. Meiosis I is a reduction division resulting in two haploid daughter nuclei from one diploid cell. Meiosis II is very similar to mitosis, where the genetic material in each of the two meiosis I daughter nucleus is halved to form four gametes.

The stages involved in mitosis, meiosis I and meiosis II are:

• Prophase
• Metaphase
• Anaphase
• Telophase

The names of these stages can be memorised by the phrase PMAT.

Meiosis I

There are six stages to meiosis I:

1. Early prophase I. This is identical to mitosis early prophase where:
   1. chromosomes become visible due to condensation of chromatin
2. Middle prophase I.
   1. homologous chromosomes pair up in synapsis to form bivalents
   2. centrioles move to opposite poles of the cell
3. Late prophase I.
   1. nuclear envelope dissolves
   2. nucleolus completely disappears
   3. crossing over of chromatids within bivalents may occur to form a chiasma (pl. chiasmata)
   4. spindle is formed at end
4. Metaphase I.
   1. bivalents line up along equator of spindle, attached to centromeres
5. Anaphase I.
   1. unlike mitosis, centromeres do not divide
   2. whole chromosomes move to spindle of cell, centromeres first, pulled by microtubules.
6. Telophase I
   1. spindle dissolves
   2. nuclear envelopes form around the two daughter nuclei
   3. a nucleolus reforms in each of the two daughter nuclei
   4. cytokinesis may occur - most animal cells will undergo cytokinesis but many plant cells pass into meiosis II without doing so.

Meiosis II

Meiosis II is very similar to mitosis, with the exception that four haploid cells are formed. The stages listed below occur in both of the meiosis I daughter cells:

1. Prophase II.
   1. nuclear envelope dissolves
   2. nucleolus disperses as chromosomes become visible due to condensation of chromatin
   3. centrioles replicate and move to opposite poles of the cell
2. Metaphase II.
   1. spindle formed
   2. chromosomes line up across the equator of the spindle
3. Anaphase II
   1. centromeres divide
   2. individual chromatids are moved the poles of the cell by microtubules, centromeres first
4. Telophase II
   1. spindle dissolves
   2. nuclear envelopes form around the four daughter nuclei
   3. a nucleolus reforms in each of the four daughter nuclei
   4. cytokinesis

See also

• Mitosis
• Sexual reproduction