chiasma

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chiasma

 [ki-az´mah] (pl. chias´mata) (L.; Gr.)
in genetics, the points at which members of a chromosome pair are in contact during the prophase of meiosis and because of which recombination, or crossing over, occurs on separation. See also chiasma formation.

chi·asm

(kī'azm),
1. An intersection or crossing of two lines.
2. In anatomy, a decussation or crossing of two fibrous bundles, such as tendons, nerves, or tracts.
3. In cytogenetics, the site at which two homologous chromosomes make contact (thus appearing to be crossed), enabling the exchange of genetic material during the prophase stage of meiosis.
Synonym(s): chiasma [TA]
[G. chiasma]

chiasma

(kī-ăz′mə) also

chiasm

(kī′ăz′əm)
n. pl. chias·mata (-mə-tə) or chias·mas also chi·asms
1. Anatomy A crossing or intersection of two tracts, as of nerves or ligaments.
2. Genetics The point of contact between paired chromatids during meiosis, resulting in a cross-shaped configuration and representing the cytological manifestation of crossing over.

chi·as′mal, chi·as′mic (-măt′ĭk), chi′as·mat′ic (-măt′ĭk) adj.

chi·asm

(kīazm)
1. An intersection or crossing of two lines.
2. anatomy a decussation or crossing of two fibrous bundles, such as tendons, nerves, or tracts.
Synonym(s): chiasma.
3. cytogenetics the site at which two homologous chromosomes make contact (thus appearing to be crossed), enabling the exchange of genetic material during the prophase stage of meiosis.
Synonym(s): Budd syndrome.
[G. chiasma]

chiasma

1. The intersection and partial crossing of the optic nerves behind the eyes within the skull. The fibres on the outer halves of each optic nerve do not cross over; those on the inner halves of each nerve do. Also known as the optic chiasm.
2. The site at which a pair of homologous chromosomes exchange material during MEIOSIS.

chiasma

(pl. chiasmata) the cross-shaped configuration produced during CROSSING OVER; for example, between CHROMATIDS in MEIOSIS.

chi·asm

, chiasma (kīazm, kī-azmă)
In anatomy, decussation or crossing of two fibrous bundles, such as tendons, nerves, or tracts.
[G. chiasma]
References in periodicals archive ?
Recombination and chiasmata: a few intriguing discrepancies.
In any organism that is or behaves like a diploid, chromosome pairing is important for two reasons: It insures that crossing-over - i.e., the reciprocal exchange of parts of the chromatids of the homologous chromosomes - can take place, and it ensures that bivalents are formed from the homologous parental chromosomes and are held together by chiasmata until first meiotic division.
Janssens (1924) extended his meticulous observations to insects and especially, grasshoppers in a monumental 224-page paper, which not only reinforced all previous observations of chiasmata, but has to be considered one of the XXth century's masterpieces of cytogenetic observation and illustration.
chiasma (pl., chiasmata): Point of physical exchange of equal segments of adjacent nonsister chromatids during the bivalent stage of meiosis.
The occurrence of about half rod bivalents is evidence that there may be insufficient chiasmata to hold quadrivalents together, as is the case in alfalfa (McCoy and Bingham, 1988).
selection acts 1 On a molecular level, a piece of DNA, usually not larger than a single chromosome (B- chromosomes, however, can pair among themselves to form a chiasmata) 2 On a whole organismal level 3 On a cellular level 4 On groups of conspecifics that intermingled together 5 On different ramets of the same genet 6 On multiunit entities 7 On the whole organismic level No.
In addition, however, it needs to be stressed that both processes must have profound influences on genetic recombination and segregation by changing the number and nature of linkage groups (interchromosome modification), while the known marked differences in frequency and distribution of chiasmata in metacentrics as compared with acro- or telocentrics (Mattsson, 1971; Bideau, 1990, 1993) should result in different patterns of intrachromosome recombination.
Diakinetic nuclei were examined for the frequency of chiasmata (bivalent/trivalent formation) and for frequency and incidence of crossing over within the inverted region of pericentric inversion heterozygotes.
He studied the effect of external environmental factors on the frequency of recombination in grasshoppers, first by looking at the influence of temperature on chiasmata frequencies and later at the consequences of x-irradiation.
Chiasmata An intersection between two fibrous bundles.
In these cells, most of the bivalents possessed an interstitial chiasma with a cross configuration, with the exception of bivalents 1 and 2 that showed two chiasmata, assuming a ring configuration.