Editing Prophase (section)

== Meiotic prophase ==
[[Meiosis]] involves two rounds of [[chromosome segregation]] and thus undergoes prophase twice, resulting in prophase I and prophase II.<ref name=":3" /> Prophase I is the most complex phase in all of meiosis because [[homologous chromosome]]s must pair and exchange [[Nucleic acid sequence|genetic information]].<ref name=":2" />{{rp|98}} Prophase II is very similar to [[Mitosis|mitotic]] prophase.<ref name=":3" />

=== Prophase I ===
Prophase I is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. In addition to the events that occur in [[Mitosis|mitotic]] prophase, several crucial events occur within these phases such as pairing of [[homologous chromosome]]s and the reciprocal [[Chromosomal crossover|exchange of genetic material]] between these [[homologous chromosome]]s. Prophase I occurs at different speeds dependent on [[species]] and [[sex]]. Many species arrest [[meiosis]] in diplotene of prophase I until [[ovulation]].<ref name=":2" />{{rp|98}} In humans, decades can pass as [[oocyte]]s remain arrested in prophase I only to quickly complete meiosis I prior to [[ovulation]].<ref name=":3" />

==== Leptotene ====
{{main|Leptotene stage}}
In the first stage of prophase I, leptotene (from the Greek for "delicate"), [[chromosome]]s begin to condense. Each chromosome is in a [[Ploidy|diploid]] state and consists of two [[sister chromatids]]; however, the [[chromatin]] of the [[sister chromatids]] is not yet condensed enough to be resolvable in [[microscopy]].<ref name=":2" />{{rp|98}} [[Homology (biology)|Homologous]] regions within [[homologous chromosome]] pairs begin to associate with each other.<ref name=":0" />

==== Zygotene ====
In the second phase of prophase I, zygotene (from the Greek for "conjugation"), all maternally and paternally derived [[chromosome]]s have found their [[Homologous chromosome|homologous]] partner.<ref name=":2" />{{rp|98}} The homologous pairs then undergo synapsis,a process by which the [[synaptonemal complex]] (a proteinaceous structure) aligns corresponding regions of [[Nucleic acid sequence|genetic information]] on maternally and paternally derived non-sister [[chromatid]]s of [[homologous chromosome]] pairs.<ref name=":2" />{{rp|98}}<ref name=":3" />  The paired homologous chromosome bound by the [[synaptonemal complex]] are referred to as [[Bivalent (genetics)|bivalents]] or tetrads.<ref name=":1" /><ref name=":2" />{{rp|98}} [[Allosome|Sex (X and Y) chromosomes]] do not fully synapse because only a small region of the chromosomes are homologous.<ref name=":2" />{{rp|98}}

The [[nucleolus]] moves from a central to a peripheral position in the [[Cell nucleus|nucleus]].<ref>{{cite journal | vauthors = Zickler D, Kleckner N | title = The leptotene-zygotene transition of meiosis | journal = Annual Review of Genetics | volume = 32 | pages = 619–97 | date = 1998 | pmid = 9928494 | doi = 10.1146/annurev.genet.32.1.619 }}</ref>

==== Pachytene ====
The third phase of prophase I, pachytene (from the Greek for "thick"), begins at the completion of synapsis.<ref name=":2" />{{rp|98}} [[Chromatin]] has condensed enough that [[chromosome]]s can now be resolved in [[microscopy]].<ref name=":1" /> Structures called recombination nodules form on the [[synaptonemal complex]] of [[Bivalent (genetics)|bivalents]]. These recombination nodules facilitate [[Chromosomal crossover|genetic exchange]] between the non-sister chromatids of the [[synaptonemal complex]] in an event known as [[Chromosomal crossover|crossing-over]] or genetic recombination.<ref name=":2" />{{rp|98}} Multiple recombination events can occur on each bivalent. In humans, an average of 2-3 events occur on each chromosome.<ref name=":4" />{{rp|681}}

==== Diplotene ====
In the fourth phase of prophase I, diplotene (from the Greek for "twofold"), [[Chromosomal crossover|crossing-over]] is completed.<ref name=":2" />{{rp|99}}<ref name=":1" /> [[Homologous chromosome]]s retain a full set of genetic information; however, the [[homologous chromosome]]s are now of mixed maternal and paternal descent.<ref name=":2" />{{rp|99}} Visible junctions called chiasmata hold the [[homologous chromosome]]s together at locations where recombination occurred as the [[synaptonemal complex]] dissolves.<ref name=":3" /><ref name=":2" />{{rp|99}} It is at this stage where meiotic arrest occurs in many [[species]].<ref name=":2" />{{rp|99}}

==== Diakinesis ====
In the fifth and final phase of prophase I, diakinesis (from the Greek for "double movement"), full chromatin condensation has occurred and all four [[sister chromatids]] can be seen in [[Bivalent (genetics)|bivalents]] with [[microscopy]]. The rest of the phase resemble the early stages of mitotic [[prometaphase]], as the meiotic prophase ends with the [[spindle apparatus]] beginning to form, and the [[nuclear membrane]] beginning to break down.<ref name=":1" /><ref name=":2" />{{rp|99}}

=== Prophase II ===
Prophase II of [[meiosis]] is very similar to prophase of [[mitosis]]. The most noticeable difference is that prophase II occurs with a [[Ploidy|haploid]] number of [[chromosome]]s as opposed to the [[Ploidy|diploid]] number in mitotic prophase.<ref name=":3" /><ref name=":1" /> In both [[Cell (biology)|animal]] and [[plant cell]]s chromosomes may de-condense during [[telophase]] I requiring them to re-condense in prophase II.<ref name=":2" />{{rp|100}}<ref name=":1" /> If chromosomes do not need to re-condense, prophase II often proceeds very quickly as is seen in the [[model organism]] [[Arabidopsis]].<ref name=":1" />