CHIASMA
Chiasma (from the Greek chiasma - decussation) is the region, where two homologous chromosomes exchange their hereditary material during meiosis. Chiasma has the characteristic shape of decussation of conjugating chromosomes. In chiasma exchange of regions of chromatids occurs. The exchange is the result of ruptures and reunions. Two of the four fibers, which are present in every chiasma, participate in them. As the result, genes of one chromosome turn out to be joined with genes of the other chromosome, and this causes new genic combinations in the chromatids, which are being formed. This process is called crossing-over.