chiasma

Chiasma, from the Greek chi meaning cross, is a term used in biology to denote a crossing or crossing-like structure. It is most widely encountered in genetics, where a chiasma refers to the cross-shaped connection between homologous chromosomes during meiosis, at which point genetic material is exchanged between non-sister chromatids in a process known as crossing over. Chiasmata typically become visible during diplotene of prophase I, when the synaptonemal complex has disassembled but the homologs remain held together at crossover sites. The resulting recombinant chromatids contribute to genetic diversity among gametes. Chiasmata can be terminal, near the ends of chromosome arms, or interstitial, located more toward the middle.

In anatomy, the term is used more generally for any cross-shaped intersection of structures. The best-known example is the optic chiasm (optic chiasma), an X-shaped region at the base of the brain where optic nerve fibers from the two eyes partially cross. This arrangement enables the integration of visual information from both eyes and contributes to binocular vision.

Across disciplines, chiasma denotes a crossing point or arrangement that resembles the letter X. While most common in genetics and neuroanatomy, the concept has broader applicability to other crossing structures described as chiasmata in scientific literature. Understanding chiasmata helps explain mechanisms of genetic recombination and the anatomical organization of certain neural pathways.