Synapsis
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Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis.[1] It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-RNA complex called the synaptonemal complex.[2] While autosomes undergo synapsis during meiosis, sex chromosomes often remain unpaired.[3]
When the non-sister chromatids intertwine, segments of chromatids with similar sequence may break apart and be exchanged in a process known as genetic recombination or "crossing-over". This exchange produces a chiasma, a region that is shaped like an X, where the two chromosomes are physically joined. At least one chiasma per chromosome appears to be necessary to stabilise bivalents along the metaphase plate during separation. The crossover of genetic material also provides a possible defence against 'chromosome killer' mechanisms, by removing the distinction between 'self' and 'non-self' through which such a mechanism could operate. A further consequence of recombinant synapsis is to increase genetic variability within both offspring and the wider population. Repeated recombination also has the general effect of allowing genes to move independently of each other through the generations, allowing for the independent concentration of beneficial genes and the purging of the detrimental.
The central function of synapsis is therefore the identification of homologues by pairing, an essential step for a successful meiosis, but the chiasma formation that takes place within synapsis has consequences at many levels, from cellular mechanics through to wide-ranging impacts upon evolution itself.
UC Berkeley video of chromosome end migration and match assessment during prophase
[edit] References
- ^ McKee B (2004). "Homologous pairing and chromosome dynamics in meiosis and mitosis". Biochim Biophys Acta 1677 (1-3): 165–80. doi:10.1016/j.bbaexp.2003.11.017. PMID 15020057.
- ^ Revenkova E, Jessberger R (2006). "Shaping meiotic prophase chromosomes: cohesins and synaptonemal complex proteins". Chromosoma 115 (3): 235–40. doi:10.1007/s00412-006-0060-x. PMID 16518630. http://www.springerlink.com/content/fm26j18850w2762t/fulltext.pdf.
- ^ 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 (2006). "Sex chromosomes, synapsis, and cohesins: a complex affair". Chromosoma 115 (3): 250–9. doi:10.1007/s00412-006-0059-3. PMID 16544151. http://www.springerlink.com/content/1683677374l40532/fulltext.pdf.
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