Synapsis

Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. [cite journal |author=McKee B |title=Homologous pairing and chromosome dynamics in meiosis and mitosis |journal=Biochim Biophys Acta |volume=1677 |issue=1-3 |pages=165–80 |year=2004 |pmid=15020057] It is a form of chromosomal crossover. Synapsis takes place during prophase I. When homologous chromosomes synapse, they come closer together until they are connected by a protein complex called the synaptonemal complex, which contains central and lateral elements. [cite journal |author=Revenkova E, Jessberger R |title=Shaping meiotic prophase chromosomes: cohesins and synaptonemal complex proteins |url=http://www.springerlink.com/content/fm26j18850w2762t/fulltext.pdf |journal=Chromosoma |volume=115 |issue=3 |pages=235–40 |year=2006 |pmid=16518630 |doi=10.1007/s00412-006-0060-x] While autosomes undergo synapsis during meiosis sex chromosomes usually remain unpaired. [cite journal |author=Page J, de la Fuente R, Gómez R, Calvente A, Viera A, Parra M, Santos J, Berríos S, Fernández-Donoso R, Suja J, Rufas J |title=Sex chromosomes, synapsis, and cohesins: a complex affair |url=http://www.springerlink.com/content/1683677374l40532/fulltext.pdf |journal=Chromosoma |volume=115 |issue=3 |pages=250–9 |year=2006 |pmid=16544151 |doi=10.1007/s00412-006-0059-3]

When the non-sister chromatids intertwine, segments of chromatids with the same sequence break apart at and are exchanged in a process known as genetic recombination or "crossing-over". Recombination exchanges genetic material between homologous chromosomes and increases the genetic variability of the offspring. This exchange produces a chiasma, a region that is shaped like an X, where the two chromosomes are physically joined.

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