<P> Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis . It allows matching - up of homologous pairs prior to their segregation, and possible chromosomal crossover between them . Synapsis takes place during prophase I of meiosis . When homologous chromosomes synapse, their ends are first attached to the nuclear envelope . These end - membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired . Then the intervening regions of the chromosome are brought together, and may be connected by a protein - RNA complex called the synaptonemal complex . Autosomes undergo synapsis during meiosis, and are held together by a protein complex along the whole length of the chromosomes called the synaptonemal complex . Sex chromosomes also undergo synapsis; however, the synaptonemal protein complex that holds the homologous chromosomes together is only present at one end of each sex chromosome . </P> <P> This is not to be confused with mitosis . Mitosis also has prophase, but does not ordinarily do pairing of two homologous chromosomes . </P>

Which term refers to the pairing of homologous chromosomes during meiosis