DO CLUSTERS OF REPLICATION UNITS IN THE MAMMALIAN-CELLS EXIST

Evidence against the existence of clusters of synchronously operating units in human and Chinese hamster cells has been presented by means of the DNA fiber autoradiography. The analysis of DNA molecule autoradiographs obtained by different labelling protocols shows that the appearance of tandemly arrayed units may be an artifact associated with fibre autoradiography. The data obtained indicate that the method of measuring centre-to-centre distances previously used to estimate replication unit size is not reliable. An attempt to define the heterogeneity of replication unit size shows that the distances between the centres of adjacent replication units vary from 5 to 500 μm, average value 120 μm. © 1979.