Abstract
In the kangaroo rat ( D. ordii) the replication of the nuclear satellite DNAs (comprising 60% of the total nuclear DNA) occupies the second half of the S period. Some of the results indicate that the different satellites are replicated in a fixed sequence within the second half of S. Auto-radiographic studies show that the DNA replicated in first half of S (non-satellite) is located in many of the chromosomes only in the longer arms. The reciprocal pattern is obtained with labelling in the second half of S. In view of the replication of satellite DNA in the second half of S, this leads to the conclusion that the shorter arms of chromosomes (plus the kinetochore) are largely if not entirely composed of satellite DNA. At least one chromosome may contain only satellite DNA. The heterochromatin pattern (Giemsa staining) of metaphase chromosomes is essentially identical with the labelling pattern of the second half of S. We conclude that the satellite DNAs of D. ordii are non-randomly distributed within the chromosome and are responsible for the pattern of heterochromatin of metaphase chromosomes.
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