Replication in chromosomes

DNA fibre autoradiography of highly polytenized nuclei in salivary glands of larvae reveals two distinct types of active replicons. Type I replicons are longer (mean size=64 μm), have a very high rate of fork migration (average rate=0.95 μm/min) and generally occur in large arrays often extending over several thousand μm. In contrast, the type II replicons are smaller (mean size= 20 μm), slow replicating (average rate=0.07 μm/min) and occur in short arrays containing only a few closely spaced active replicons. Evidence is presented that type I replicons are active in the early S and type II in the late S. Observations on autoradiographic labelling of partially lysed polytene chromosomes provide evidence for a lack of temporal and spatial agreement in the activation of origin points in homologous regions of the lateral polytene strands; these observations also suggest local variations in levels of polyteny within a chromosome. On the basis of this and other available information on replication in polytene chromosomes the possible roles of the two replicon types in the generation of the different H-thymidine labelling patterns of polytene chromosomes are discussed.