noun, plural chi·as·mas, chi·as·ma·ta [kahy-az-muh-tuh] /kaɪˈæz mə tə/.

  1. Anatomy. a crossing or decussation, as that of the optic nerves at the base of the brain.
  2. Cell Biology. a point of overlap of paired chromatids at which fusion and exchange of genetic material take place during prophase of meiosis.

noun plural -mas, -mata (-mətə) or -asms

  1. cytology the cross-shaped connection produced by the crossing over of pairing chromosomes during meiosis
  2. anatomy the crossing over of two parts or structures, such as the fibres of the optic nerves in the brain

“a crossing,” 1832, medical Latin, from Greek khiasma “two things placed crosswise,” which is related to khiasmos (see chiasmus). In cytology from 1911.

n. pl. chi•as•mas

  1. A crossing or intersection of two tracts, as of nerves or ligaments.
  2. The point of contact between paired chromatids during meiosis, resulting in a cross-shaped configuration and representing the cytological manifestation of crossing over.
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