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 ?
Next, Shakespeare splits this basic chiastic arrangement of parallelisms and forms a border surrounding the central chiasm of the overall system:
A basic scheme might be notated as ABCBA, with the C level representing the apex; however, this type of chiasm may actually contain any number of levels and is more directly related to the biblical forms than the first two examples (Renaissance writers often compose passages of successive couplets, or phrases employing parison, which can mimic some of the earlier ABA examples mentioned here).
The following system is an ABCBA chiasm linked to an ABA chiasm with a typical Shakespearean catalogue embedded in the structure (the word sin illustrates rhetorical repetition, but it does not define the structural spine of the chiasm):
In this line, the words "love" and "hate" form a chiasm with "brawling" and "loving," as well as simultaneously forming yet another chiasm with line 175, the previous verse line in the text (as seen below).
More specifically, the basic process of composing a passage according to complex chiastic principles begins by outlining a simple chiasm (such as the basic inclusion shown above), and then expanding each level with yet another chiasm, catalogue, periodic sentence, parallelism, or any other type of structure that addresses the original concept.
Helena, a virgin; chiasm 2] B2: were made of B2: is metal to make A2: virgins.
As a pattern of composition, complex chiasmus is also a very fast and economical way to write (once the first half of a chiasm is written, the remaining portion has almost written itself), which would permit an author to generate a much greater output of material than might otherwise be achieved without the device.
Thousands of years have passed since the first writer of the ancient Mediterranean world composed the first complex chiasm, a form that subsequent generations explored and developed to an elaborate degree (though apparently falling into general disuse in later generations).