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the side by side pairing of homologous maternal and paternal chromosomes at the start of meiosis
the act of grouping things or people in pairs

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/08/29 23:11:03」(JST)

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Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. 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.^[1] While autosomes undergo synapsis during meiosis, sex chromosomes often remain unpaired.^[2]

This is not to be confused with mitosis. Mitosis also has prophase, but does not ordinarily do pairing of two homologous chromosomes.^[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 often 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 the offspring. 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.

Following synapsis, a type of recombination referred to as synthesis dependent strand annealing (SDSA) occurs frequently. SDSA recombination involves information exchange between paired non-sister homologous chromatids, but not physical exchange. SDSA recombination does not cause crossing-over. Both the non-crossover and crossover types of recombination function as processes for repairing DNA damage, particularly double-strand breaks (see Genetic recombination).

The central function of synapsis is therefore the identification of homologues by pairing, an essential step for a successful meiosis. The processes of DNA repair and chiasma formation that take place following synapsis have consequences at many levels, from cellular survival through to impacts upon evolution itself.

References

^ Revenkova E, Jessberger R (2006). "Shaping meiotic prophase chromosomes: cohesins and synaptonemal complex proteins" (PDF). Chromosoma. 115 (3): 235–40. doi:10.1007/s00412-006-0060-x. PMID 16518630.
^ 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" (PDF). Chromosoma. 115 (3): 250–9. doi:10.1007/s00412-006-0059-3. PMID 16544151.
^ 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.

External links

UC Berkeley video of chromosome end migration and match assessment during prophase

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English Journal

Sparse reconstruction of brain circuits: Or, how to survive without a microscopic connectome.

da Costa NM, Martin KA.SourceInstitute of Neuroinformatics UZH/ETHZ, Wintherturerstrasse 190, 8057 Zurich, Switzerland. Electronic address: ndacosta@ini.phys.ethz.ch.
NeuroImage.Neuroimage.2013 Oct 15;80:27-36. doi: 10.1016/j.neuroimage.2013.04.054. Epub 2013 Apr 25.
Inside one voxel of a cubic millimeter of neocortex, fifty to hundred thousand neurons use 4km of axonal cable to form three to fifteen hundred million synapses with each other. While in the human, such voxel is a small fragment of a cortical area, in the mouse an entire cortical area, like the prim
PMID 23624494

Quantitative analysis of ribbons, vesicles, and cisterns at the cat inner hair cell synapse: Correlations with spontaneous rate.

Kantardzhieva A, Liberman MC, Sewell WF.SourceEaton-Peabody Laboratory, Department of Otology and Laryngology, Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts, 02114.
The Journal of comparative neurology.J Comp Neurol.2013 Oct 1;521(14):3260-71. doi: 10.1002/cne.23345.
Cochlear hair cells form ribbon synapses with terminals of the cochlear nerve. To test the hypothesis that one function of the ribbon is to create synaptic vesicles from the cisternal structures that are abundant at the base of hair cells, we analyzed the distribution of vesicles and cisterns around
PMID 23787810

Synaptic and extrasynaptic location of the receptor tyrosine kinase met during postnatal development in the mouse neocortex and hippocampus.

Eagleson KL, Milner TA, Xie Z, Levitt P.SourceZilkha Neurogenetic Institute, Keck School of Medicine of USC, Los Angeles, California, 90033; Department of Cell & Neurobiology, Keck School of Medicine of USC, Los Angeles, California, 90033.
The Journal of comparative neurology.J Comp Neurol.2013 Oct 1;521(14):3241-59. doi: 10.1002/cne.23343.
MET, a replicated autism risk gene, encodes a pleiotropic receptor tyrosine kinase implicated in multiple cellular processes during development and following injury. Previous studies suggest that Met modulates excitatory synapse development in the neocortex and hippocampus, although the underlying m
PMID 23787772

Japanese Journal

Controlling the stoichiometry and strand polarity of a tetramolecular G-quadruplex structure by using a DNA origami frame.

Rajendran Arivazhagan,Endo Masayuki,Hidaka Kumi,Lan Thao Tran Phong,Mergny Jean-Louis,Sugiyama Hiroshi
Nucleic acids research 41(18), 8738-8747, 2013-10
… Such a quadruplex formation allowed the DNA synapsis without disturbing the duplex regions of the participating sequences, and resulted in an X-shaped structure that was monitored by atomic force microscopy. …
NAID 120005347132

Infertility Associated with Meiotic Failure in the tremor Rat (tm/tm) is Caused by the Deletion of Spermatogenesis Associated 22

ISHISHITA Satoshi,INUI Toshihide,MATSUDA Youichi [他],Serikawa Tadao,Kitada Kazuhiro
Experimental Animals 62(3), 219-227, 2013
… Spermatogenesis associated 22 (Spata22) has been shown to be a vertebrate-specific gene essential for the progression of meiosis through prophase I and completion of chromosome synapsis and meiotic recombination using a mouse repro42 mutant carrying an N-ethyl-N-nitrosourea (ENU)-induced nonsense mutation in Spata22. …
NAID 130003362962

Chromosome stickiness leading to desynapsis and sterility in safflower

Srivastava Preeti
Chromosome Botany 7(1), 37-41, 2012
… The variant plants scored were found to have medium type desynapsis as demonstrated by enhanced frequency of univalents per cell as opposed to bivalents in all the PMCs analyzed, a convincing indication of failure of synapsis between homologous pairs of chromosomes. …
NAID 130002111697