Synapsis - Wikipedia Synapsis Not to be confused with synapse, synapsid, or synopsis. For the plant genus, see Synapsis ( plant ). For the beetle genus, see Coprini. 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. Autosomes undergo synapsis during meiosis, and are held together by a protein complex along the whole length of the chromosomes called the synaptonemal complex. Sex chromosomes also undergo synapsis ; however, the synaptonemal protein complex that holds the homologous chromosomes together is only present at one end of each sex chromosome. This is not to be confused with mitosis. Mitosis also has prophase, but does not ordinarily do pairing of two homologous chromosomes. 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 Jump up ^ Revenkova E, Jessberger R ( 2006 ). `` Shaping meiotic prophase chromosomes : cohesins and synaptonemal complex proteins '' ( PDF ). Chromosoma. 115 ( 3 ) : 235 -- 40. PMID 16518630. doi : 10.1007 / s00412 - 006 - 0060 - x. CS1 maint : Uses authors parameter ( link ) Jump up ^ 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. PMID 16544151. doi : 10.1007 / s00412 - 006 - 0059 - 3. CS1 maint : Multiple names : authors list ( link ) Jump up ^ McKee B ( 2004 ). `` Homologous pairing and chromosome dynamics in meiosis and mitosis ''. Biochim Biophys Acta. 1677 ( 1 - 3 ) : 165 -- 80. PMID 15020057. doi : 10.1016 / j. bbaexp. 2003.11. 017. External links Look up synapsis in Wiktionary, the free dictionary. UC Berkeley video of chromosome end migration and match assessment during prophase This cell biology article is a stub. You can help Wikipedia by expanding it. Retrieved from `` https://en.wikipedia.org/w/index.php?title=Synapsis&oldid=807090876 '' Categories : Cell biology Cytogenetics Cell biology stubs Hidden categories : CS1 maint : Uses authors parameter CS1 maint : Multiple names : authors list All stub articles Talk About Wikipedia Español Français Hrvatski Português Русский தமிழ் 中文 Edit links This page was last edited on 25 October 2017, at 21 : 01. About Wikipedia

Question: when does synapsis of homologous chromosomes occur in mitosis
Answer: prophase I of meiosis; during prophase I of meiosis; during prophase I of meiosis