At the end of meiotic prophase I the chromosomes have paired, and sister chromatids are visible. The paired chromosomes form tetrads. This requires condensation of the chromosomes and pairing of homologs in a chromosome pair. Usually, formation of the tetrad is accompanied by synapsis between nonsister chromatids of a homologous pair, leading to subsequent crossing over and a reciprocal exchange of genetic material. The individual chromosomes in the tetrad are later separated in anaphase I. It is only in meiosis II that the sister chromatids are separated and form individual chromosomes.