Abstract
Tritium autoradiography was used to determine the initial and terminal sequences of DNA synthesis in chromosomes of two marsupial species with morphologically similar chromosomes. Statistical analysis of grain counts over chromosome segments or arms showed that in each species there was no significant difference in amount of synthesis by homologous autosomal regions, while one X chromosomes in females was always delayed in initiating and terminating synthesis. This chromosome was always more heavily condensed than its partner. Heterogeneity analysis showed that cells followed a definite pattern of synthesis, and the course of synthesis in each chromosome could be described graphically. Using a mixed culture technique to compare synthesis pattern between animals of different sex and of different species, it was found that this pattern was similar in homologous chromosomes o of the male and female, and also in morphologically similar chromosomes of the different species. It was concluded that the synthesis pattern is a species constant, and that this pattern differs in the two species only in chromosome 1, in which it accompanies a chromosome rearrangement.
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