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The process by which a diploid (2n) cell forms four haploid (1n) gametes. Meiosis differs from mitosis as meiosis I involves chromosomal content being recombinantly halved.
Overview of meiosis
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:
The names of these stages can be memorised by the phrase PMAT.
There are four stages to meiosis I:
- Prophase. This is much longer than the mitotic prophase and is divided into 5 substages:
- Leptotene. Chromosomes organise axial elements and acquire recombination proteins. Homologues pair ready for synapsis.
- Zygotene. Crossing over sites are designated, chromosomes begin and complete synapsis. Axial elements align to form synaptonemal complex.
- Pachytene. Chromosomes are thicker due to pairing and contraction.
- Diplotene. Synaptonemal complex degrades and 4 chromatids become visible. In some organisms (such as goats) this stage is prolonged into dictyotene.
- Diakinesis. Chromosomes continue to thicken due to coiling, nuclear envelop disappears, kinetochores and spindle develop.
- Bivalents line up along equator of spindle, attached to centromeres.
- unlike mitosis, centromeres do not divide.
- whole chromosomes move to spindle of cell, centromeres first, pulled by microtubules.
- spindle dissolves
- nuclear envelopes form around the two daughter nuclei
- a nucleolus reforms in each of the two daughter nuclei
- Cytokinesis may occur - most animal cells will undergo Cytokinesis but many plant cells pass into meiosis II without doing so.
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:
- Prophase II.
- nuclear envelope dissolves
- nucleolus disperses as chromosomes become visible due to condensation of chromatin
- centrioles replicate and move to opposite poles of the cell
- Metaphase II.
- spindle formed
- chromosomes line up across the equator of the spindle
- Anaphase II
- centromeres divide
- individual chromatids are moved the poles of the cell by microtubules, centromeres first
- Telophase II
- spindle dissolves
- nuclear envelopes form around the four daughter nuclei
- a nucleolus reforms in each of the four daughter nuclei
- Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4