Chiasmata are specialized chromatin structures that link homologous chromosomes together until anaphase I (Figs. 45.1 and 45.10). They form at sites where programmed DNA breaks generated by Spo11 undergo the full recombination pathway to generate crossovers.

Where does chiasma occur?

The chiasmata become visible during the diplotene stage of prophase I of meiosis, but the actual "crossing-overs" of genetic material are thought to occur during the previous pachytene stage.

How is the chiasmata formed?

In many species, chiasmata (the physical attachments between homologous chromosomes) are formed after the recognition of homologous chromosomes (pairing), the close association of paired chromosomes by the synaptonemal complex (SC), and the reciprocal exchange of sequences by the homologous recombination (HR) process.