Effects of temperature on the replication of chromosomal DNA of Xenopus laevis cells.

DNA fibre autoradiography has been used to study the effects of temperature on the replication of chromosomal DNA of Xenopus laevis cells in tissue culture at 18, 23 and 28 degrees C. Pulse/stepdown labelling shows that the DNA replicates bidirectionally. Origin-to-origin distances (initiation intervals) vary, but the range of and the mean initiation intervals at all three temperatures are much the same. The mean interval between initiation points is of the order of 60 to 66 microns. Staggering of initiation is evident at all three temperatures. Evidence against the existence of replication termini is provided. The rates of progress of DNA replication forks are 6 microns/h at 18 degrees C, 10 microns/h at 23 degrees C and 16 microns/h at 28 degrees C.