Chiasmata

Chiasma (plural chiasmata) is a cytogenetic structure formed at the sites where crossing-over occurs between non-sister chromatids of homologous chromosomes during meiosis. Under light microscopy, chiasmata appear as X-shaped regions that persist after the synaptonemal complex disassembles, linking homologous chromosomes at late prophase I and into metaphase I.

Formation and timing: During prophase I, homologous chromosomes pair via synapsis and exchange DNA segments through crossing-over, typically during the pachytene stage. The exchange creates a physical cross—the chiasma—that holds the homologs together as they condense and align for segregation. When diplotene occurs, the synaptonemal complex dissolves, but the chiasmata remain as visible connections until the homologs separate at anaphase I.

Structure and variation: A bivalent may have multiple chiasmata; there can be single or several crossovers per homologous pair. Chiasmata may be terminal (near chromosome ends) or interstitial (closer to the middle). The number and position of chiasmata influence recombination frequencies and are used in genetic mapping (centimorgans).

Significance: Chiasmata ensure proper segregation of homologous chromosomes and create genetic diversity by shuffling alleles between chromatids. They are a key feature of meiosis, and their observation informs studies of recombination, chromosome structure, and fertility.

Etymology: The term chiasma derives from Greek for “crossing.”