Meiosis

Meiosis I & II

In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. DNA replication precedes the start of meiosis I

The phases of meiosis I & II

Prophase I

• Homologous chromosomes pair and form synapses, a step unique to meiosis.
• The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent.
• The bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.

Prometaphase I

• The nuclear membrane disappears.
• One kinetochore forms per chromosome rather than one per chromatid
• The chromosomes attached to spindle fibers begin to move.

Metaphase I

• Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. (The orientation is random, with either parental homologue on a side so there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome).

Anaphase I

• The Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Telophase I

• Nuclear envelopes may reform, or the cell may quickly start meiosis II.

Cytokinesis

• Analogous to mitosis where two complete daughter cells form.

Meiosis Animation (created by a KLB pupil)


Meiosis II

• Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination.
• Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid.