(a) The process of association during the crossing over of chromosomes in the zygotene stage is called synapsis. (b) At the end of synapsis a synaptonemal complex is formed. This complex formed by a pair of synapsed homologous chromosomes is called a bivalent or tetrad. (c) The beginning of the diplotene stage is marked by the beginning of separation of the paired homologous chromosomes. The separation of homologous chromosomes is however not completed. They remain attached at one or more points where crossing over has occurred. These points of attachment are called chiasmata.