Grades of chromatid organisation in mitotic and meiotic chromosomes

The different grades of chromatid organisation recognised in mitotic and meiotic chromosomes are interpreted in terms of a new master/slave model, in which a linear strand of master loci has an associated slave assembly of chromomeres. These are joined by labile interchromomeric linkages, which might be short overlap segments of single-stranded DNA. Each slave assembly, or chromomere, is considered to give rise to a lateral slave loop. The model is able to explain how transitions between the four basic types of lampbrush organisation could occur at mitosis and meiosis, how the presence or absence of transitions at early first meiotic prophase may decide whether or not synapsis may take place, and why it should be restricted to two homologues at any one point. It provides a basis for understanding how precision hybrid DNA cross-over events involving single DNA molecules could be achieved in a regular, repeatable and controlled fashion, between such relatively massive structures as chromosomes, and how resolution of the primary exchange events could lead to the development of chiasmata. It also suggests a new way of thinking about the mechanisms underlying the separation and rotation of chromosome arms which occurs during the diplotene and diakinesis stages of most chiasmate species. Semi-conservative DNA replication, iso-labelling and breakage and reunion studies which suggest only one axial strand are also explicable in terms of the model, as is the occurrence of side-arm bridges, involving two visible “sub-chromatid” strands, at anaphase stages.