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 ?
In this case, the entire speech follows the pattern of a basic chiastic inclusion (ABA): Romeo's Speech (lines 175-82): A: Love and Hate (chiasms) B: Catalogue of Contraries A: Love and Hate (chiasm)
Next, by expanding each chiastic level with phrases composed of chiasms and periodic sentences (the outline of this passage generally follows the periodic sentences it contains), the full speech subsequently takes shape into a highly complex and interrelated passage: Hamlet: A: Now I am alone.
(13) Shakespeare incorporates periodic sentences of various lengths into his complex chiasms to assist in the narrative flow.
In the following example, he not only uses parallelisms but sets two parallelisms (one in each A level) into corresponding positions in the passage, creating a structural chiasm (in this case, a chiastic inclusion, or ABA) for the main body of the speech: Brackenbury (R3 1.4.76-83): Sorrow breaks seasons and reposing hours, [introduction; standard makes the night morning and the noontide night: parallelism] [basic chiasm (5)] A: Princes have but their titles for their glories, An outward honor for an inward toil, B: And for unfelt imaginations they often feel a world of restless cares A: So that between their titles and low name There's nothing differs but the outward fame.
The introductory portion, termed anacrusis among biblical scholars (Watson 150), appears as a single word, a phrase, a parallelism, or a small paragraph that propels the text into the chiasm that follows.
In biblical complex chiasmus, as well as in many of Shakespeare's forms, the structures containing the springboard and rejoinder sections are generally independent from the main body of the chiasm; however, this is not always the case.
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): King Henry the Sixth (2H6 5.1.181-90): A: Canst thou dispense with heaven for such an oath?
In the following example from The Winter's Tale, Shakespeare creates a complex, overlapping chiasm that begins with a basic arrangement of related parallelisms, which he uses to bookend a second chiasm that lives in the center of the overall passage.
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.