DNA reassociation kinetics were studied, by means of the hydroxyapatite chromatography method, for four species of Amphibians with different nuclear DNA content: Xenopus laevis (3 pg DNA per haploid genome) and Bufo bufo (7 pg) of the Anura subclass and Triturus cristatus (23 pg) and Necturus maculosus (52 pg) of the Urodela subclass. Within each subclass the two species studied were found to have about the same absolute amount of unique DNA. The differences of total nuclear DNA can be accounted for by quantitative variations of the repetitive sequence classes, at least in part due to changes in the number of copies of the various sequences. On the contrary the great difference in nuclear DNA between the two subclasses, Anura and Urodela, involves all sequence classes in parallel; the slowly reassociating fraction appears to be unique in spite of a tenfold difference in absolute amount. The dependence of reassociation kinetics on DNA fragment length for the four species indicates for all of them an interspersed organization of the various sequence classes.
Amphibians Animals