chiasma

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chiasma

(kīăz`mə): see crossing overcrossing over,
process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids.
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chiasma

[kī′az·mə]
(anatomy)
A cross-shaped point of intersection of two parts, especially of the optic nerves.
(cell and molecular biology)
The point of junction and fusion between paired chromatids or chromosomes, first seen during diplotene of meiosis.
References in periodicals archive ?
The total number of chiasmata in 45 sampled cells were 752 and an average 16.71 chiasmata per cell (Table-1).
Chiasmata and achiasmatic meiosis in African eumastacid grasshoppers.
Crossing over, the formation of chiasmata, results in additional genetic recombination, thus, increasing variability.
([double dagger]) Xta = chiasmata (points of attachment between chromosomes at meiosis resulting from crossing over).
First, the number of chiasmata observed at metaphase may be an underestimate of crossover events.
However, as stated above, chromosomes have only few morphological hallmarks, and the only features that can routinely be assessed at metaphase I of meiosis are the different chromosomal configurations (univalents, bivalents, trivalents, etc.), the number of chiasmata, and the larger structural rearrangements (e.g., translocations, inversions).
The positions of all chiasmata for a chromosome pairing are drawn, ordered, and then applied to the lists of junctions of the parents, to produce a single offspring which, if it contains any immigrant genetic material, is added to the stored individuals of the next generation.
In 1909, he published a relatively short paper (for the standards of that epoch) in which he demonstrated, through careful examination of meiotic cells (auxocytes, as he called them) of urodelans (Batrachoseps, a plethodontid salamander), that homologous chromosomes pair, and pairs of chromatids can break at corresponding points along their length and physically exchange chromosomal material producing X-like igures that he dubbed "chiasmata"; he derived a model called the chiasmatype theory (chiasmatypie, in French) (Janssens 1909).
More specifically, we observed two bivalents with two chiasmata in five diplotene nuclei and one bivalent with three chiasmata in five diplotene nuclei.
This is the site at which the chiasmata form, permitting the exchange of DNA between these regions.