Definition of chiasma in English:



  • 1Anatomy
    The X-shaped structure formed at the point below the brain where the two optic nerves cross over each other.

    • ‘The dura that invests the optic chiasma can be traced from the chiasma along the optic nerve.’
  • 2Biology
    A point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands.

    • ‘During meiosis, homologous chromosomes become physically connected by the formation of chiasmata.’
    • ‘A given factor s can affect the recombination rate r through a change in the number of chiasmata occurring during meiosis.’
    • ‘First, it generates crossovers between homologous chromosomes that mature into visible chiasmata.’
    • ‘Genetic recombination is absent and no chiasmata are formed in bivalents.’
    • ‘However, the observed differences may be conditional on the chromosomal location of the chiasmata.’


Mid 19th century: modern Latin, from Greek chiasma ‘crosspiece, cross-shaped mark’, from khiazein ‘mark with the letter chi’.