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

/chi·as·ma/ (ki-az´mah) pl. chias´mata   [L.] chiasm; 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.

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.

chiasma

[kī·az′mə] pl. chiasmata
Etymology: Gk, lines that cross
a visible connection between homologous chromosomes during the first meiotic division in gametogenesis. Chiasmata appear as X-shaped configurations during the late prophase stage and provide the means by which homologous chromosomes exchange genetic material. See also crossing over. chiasmatic, chiasmic, 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]

chiasma

pl. chiasmata [L., Gr.] chiasm; 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.

chiasma formation
the process by which a chiasma is formed; it is the cytological basis of genetic recombination, or crossing over.
chiasma syndrome
optic atrophy with bilateral hemianopia.
References in periodicals archive ?
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.
Unlike Bateson, Darlington was an early adherent to the chromosome and chiasmatype theories, and during the following years developed his theory on the evolution of genetic systems in which chromosomes, and especially chiasmata and recombination were central to the evolutionary history of organisms (Darlington 1958).
those where one arm of a metacentric formed chiasmata both with its second arm and with its homologue) were found, indicating the presence of duplicate terminal segments within single chromosomes, which in turn might suggest the presence of a pair of isochromosomes.
Results of the analysis of diakinetic nuclei from the fission/inv(4) heterozygotes were consistent with the observed suppression of chiasmata reported for the inv(4) rearrangement in S.
Through contact with one of the Schraders' students, Kenneth Cooper, he became interested in one of the central cytogenetic problems of the day, the question as to whether meiosis could proceed regularly if there were no chiasmata between homologous chromosomes at metaphase I.
All bivalents present a ring shape, evidencing the occurrence of two terminal chiasmata per bivalent, and the X chromosome constitutes an univalent during all meiosis I (Fig.
in newly enclosed flies, including areas of the cellular cortex, cell body layer of the lamina, and the first and second optic chiasmata [ILLUSTRATION FOR FIGURE 10 OMITTED].
Diakinetic nuclei from some individuals were scored for the number of elements and frequency of chiasmata.
At least in the Orthoptera, where centric fusions are a frequent mechanism of chromosome change (John & Hewitt 1968, John & Freeman 1975, John 1983), fusions produce an instantaneous shift of proximal chiasmata towards distal locations in pericentromeric regions (Bidau 1990), creating recombination-free regions.
This figure is obtained by assuming a Poisson distribution of chiasmata along the chromosome and multiplying the probabilities of recombination or no recombination in each interval [Numerically this is 0.
The random distribution of chiasmata observed within the heterochromatic pseudoautosomal segments in deer mice (Hale and Greenbaum, 1986; Sudman and Greenbaum, 1990; Hale et al.
The number of chiasmata between each of the nine bivalents is normally one, sometimes two (as observed in Figs 1a-c), frequently with interstitial localization as shown in Fig.