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SUMMARY: The uptake of the labelled precursors, thymidine, deoxyadenosine and adenine, into the nuclear and mitochondrial DNA of the cellular slime mould, Dictyostelium discoideum, at various stages of development was studied. Labelling of the different species of DNA, nuclear main-band, mitochondrial, and satellite, was analysed by CsC1 gradients using the AT-specific drug netropsin to enhance the density resolution. Constitutive and gamma ray modified uptake patterns were obtained. During early development, through late aggregation, constitutive uptake of thymidine and deoxyadenosine was exclusively into DNA at the density of mitochondrial and nuclear satellite I DNA, believed to be primarily due to mitochondrial DNA labelling. Labelled adenine was not incorporated into any DNA before early culmination. During the period of early culmination uptake of all three of these precursors into the main-band nuclear DNA increased dramatically, to considerably exceed uptake into the mitochondrial DNA. The molecular basis for these constitutive uptake patterns during development is not understood, but they appear to involve developmentally associated periods of DNA replication in some or all of the cells, possibly accompanied by changes in precursor transport and/or pool sizes that are different for the mitochondrial and nuclear DNA metabolic pathways. Early in development gamma rays had little effect on the constitutive uptake of precursor into the mitochondrial DNA but induced new dose-dependent uptake of thymidine and deoxyadenosine into the nuclear DNA. This unscheduled nuclear DNA synthesis may be a manifestation of gamma ray induced repair replication. At the early culmination stage gamma rays severely depressed the constitutive uptake of thymidine, deoxyadenosine and adenine into the nuclear DNA, to a level of about 10% at 20 krad. Higher doses induced a slight increase over this level. These changes may be due to the inhibition of the constitutive semiconservative replication by low gamma ray doses with the superposition of some induced repair replication.

Constitutive and Gamma Ray Modified Uptake of Labelled Precursors into the DNA of Dictyostelium discoideum during Development