View source for Chiasma

Chiasma or crossover refers to the visible point where paired maternal and paternal [[Homologous chromosome|homologous]] [[Homologous chromosomes|chromosomes]] join during [[Meiosis|meiosis ]]1 and then crossover. This&nbsp;[[Recombination|recombination]] occurs when a segment of one paternal [[Chromatid|chromatid]] is replaced by the [[Complementary|complementary]] form of the other maternal [[Chromatid|chromatid]]&nbsp;<ref>Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts and Peter Walter, 2008. Molecular biology of the cell. 5th edition. New York: Garland science.</ref>. 

There are 2 [[Nuclear division|nuclear divisions]] in [[Meiosis|meiosis]], and a chiasma forms in the first division of [[Meiosis|meiosis]] 1. The duplicated [[Homologues|homologs]] align with each other and swap genetic information&nbsp;[[Recombination|(recombination)]]. 

Crossover happens between 2 non-sister [[Chromatids|chromatids]]. The chiasma can be seen during the diplotene stage, at the point where the [[Homologues|homologs]]&nbsp;join in [[Prophase|prophase]] 1, when the [[Synaptonemal Complex|synaptonemal complex]] disassembles and begin&nbsp;to separate''.''&nbsp;They crucially hold the compact homologues together<ref>Alberts, B. 2008. Molecular biology of the cell. New York: Garland Science.</ref>. The frequency of crossing over varies, with the occurrence in humans being at least once in each chromosome arm. 

[[Image:Screen Shot 2013-11-27 at 15.23.19.png|Homologous pair of chromosomes showing chiasma]] 

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