Abstract
Newborn and hepatectomized rats were labelled with tritiated thymidine; specific activity of liver nuclear and nucleolar DNA was determined on purified fractions and compared for pulses varying from 2 1 2 to 120 min . Ultrastructural cytochemistry reveals that the DNA isolated with the nucleolar subfraction is contained within the perinucleolar chromatin and its intranucleolar ramifications. The specific activity of this DNA is in every case significantly lower than that of nuclear DNA, whereas nucleolar RNA shows the same activity as nuclear RNA for 12 min pulses, and twice the activity of nuclear RNA for a 45 min pulse. Kinetic analysis of the incorporation of tritiated thymidine emphasizes this lower activity of nucleolar DNA. The results are interpreted as evidence for either a shorter duration or lower rate of synthesis of nucleolar DNA as compared to nuclear DNA.
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