Chiasma (genetics)

A chiasma (Greek χίασμα chiasm " crossing", from the Greek letter Χ, Chi ) is a crossover between two chromatids of homologous chromosomes. In prophase I of meiosis occurs to shorten the chromosomes so that they are visible by light microscopy. The homologous chromosomes pair in the zygotänen phase of this stage. They lie parallel to each other ( Synapsis ), so that they lie exactly with the corresponding loci adjacent. In some places the legs of paternal and maternal chromosome arms are crossed. In this diplotene prophase Parallelkonjugation the fans and the locations of the crossovers are cytologically visible. These sites are called chiasmata, they are the result of crossing-over.