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
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.
- The nuclear membrane disappears.
- One kinetochore forms per chromosome rather
than one per chromatid
- The chromosomes attached to spindle fibers begin to
- 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).
- 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.
- Nuclear envelopes may reform, or the cell may quickly start meiosis II.
- Analogous to mitosis where two complete daughter cells form.
Meiosis Animation (created by a KLB pupil)
- 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.