chiasma

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.

[Greek khīasma, cross-piece, from khīazein, to mark with an X, from khei, khī, chi (from the letter's shape).]

The site of "crossing over," where homologous chromosomes actually exchange material during the first round of meiosis, is known as a chiasma. The plural version is chiasmata.

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.

• c. formation — the process by which a chiasma is formed; it is the cytological basis of genetic recombination, or crossing over.
• c. syndrome — optic atrophy with bilateral hemianopia.

It has been suggested that this article or section be merged into chiasm. (Discuss)

The term chiasma comes from the Greek χίασμα, which is related to the Greek letter Χ and means "crossing". It has a number of related meanings in biology. The plural is chiasmata.

Chiasma may also refer to:

• The point where nerves or ligaments cross or intersect
• The optic chiasm
• Chiasma (genetics), the point where two chromatids are intertwined (interwoven) in a cell

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