Meiosis

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[[Image:Meiosis.gif|thumb|165px|right|Meiosis Overview]]{{biology}}
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<ref>Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4</ref> 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==
There are four stages to meiosis I:
#Prophase. This is much longer than the mitotic prophase and is divided into 5 substages:
##Leptotene. [[Chromosome]]s organise axial elements and acquire recombination proteins. Homologues pair ready for synapsis.
##Zygotene. Crossing over sites are designated, chromosomes begin and complete [http://en.wikipedia.org/wiki/Synapsis 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 [http://en.wikipedia.org/wiki/Dictyotene dictyotene].
##Diakinesis. Chromosomes continue to thicken due to coiling, nuclear envelop disappears, kinetochores and spindle develop.
#Metaphase
##Bivalents line up along equator of spindle, attached to centromeres.
#Anaphase
##unlike mitosis, centromeres do not divide.
##whole chromosomes move to spindle of cell, centromeres first, pulled by microtubules.
#Telophase
##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==
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
##cytokinesis

==See also==
*[[Cell]]
*[[Sexual reproduction]]

==References==
{{reflist}}

[[Category:Articles needing RWification]]
[[Category:Cellular biology]]

Categories: Articles needing RWification, Cellular biology