The initial stage of nuclear division. In both *
mitosis and *
meiosis, the chromosomes become coiled and recoiled, and, in mitosis, the chromatids can be identified. As they shorten and thicken the distinctive features of the individual chromosomes can be identified with the light microscope. In meiosis, prophase of the first division can be divided into five substages, but there is no clear demarcation between them, the whole process being continuous. The first substage, leptotene, is the period during which shortening and thickening occurs, but, although it is known that DNA replication has occurred, the chromosomes do not appear to be divided into chromatids. During the next two substages, pachytene and zygotene, homologous chromosomes are attracted to each other and *
synapsis takes place. This is in contrast to the situation in mitotic prophase where homologues remain entirely separate from each other. The paired chromosomes continue to contract and coil around each other to form a composite structure called a bivalent. Diplotene, the fourth substage, begins as the mutual attraction between the chromosomes of the bivalents lapses and is replaced by mutual repulsion, commencing at the centromeres. The chromatids at this stage are clearly visible, and, as the chromosomes separate, it becomes apparent that they are held together at various points, *
chiasmata, where chromatids from opposite chromosomes have crossed over and are linked together. By diakinesis, the chromosomes are fully contracted. As the centromeres of the homologues continue to move apart, each pulls its attached chromatid pair with it and the regions of cross-over move towards the ends of the chromosomes (terminalization). By the end of prophase in both mitosis and meiosis, the nucleoli have dispersed and the nuclear membrane has broken down. In prophase of the second meiotic division, there is only one set of chromosomes i.e. one member only of each homologous pair. See
metaphase ,
anaphase ,
telophase.
