In vivo effects of cytosine arabinoside on deoxyribonucleic acid replication in Chinese hamster ovary cells. 2. Cytosine arabinoside affects the rate of synthesis but not the pattern of labeling of an amplified chromosomal sequence at the onset of the S period

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Abstract
The effect of [the antineoplastic drug] 1-.beta.-D-arabinofurosylcytosine (ara-C) on DNA replication in methotrexate-resistant Chinese hamster ovary [CHO] cells was examined under circumstances in which nuclear DNA synthesis could be distinguished from mitochondrial DNA synthesis. G1-arrested cells were induced to traverse G1 and enter the S phase in the presence of radiolabeled thymidine and various cocentrations of the drug. ara-C did not affect the kinetics of G1 traverse and subsequent entry into S after release from isoleucine deprivation, as measured by autoradiography. The inhibitor reduced the net rate of thymidine incorporation into nuclear DNA in a dose-dependent fashion. Autoradiography of nuclear matrix-DNA halo structures suggests that the drug inhibits nuclear thymidine incorporation by slowing chain elongation and movement of newly replicated DNA through a matrix-bound replication apparatus. Southern blot analysis of restriction digests of DNA radiolabeled in early S in the presence of ara-C indicates that the synthesis of the early-replicating amplified dihydrofolate reductase domain in these cells begins at sequences identical with those observed in cells synchronized with aphidicolin or hydroxyurea. Progressively lower concentrations of ara-C permit proportionately greater extents of the amplified unit to be replicated. Thus, ara-C apparently slows the rate of chain elongation without altering the site at which DNA replication is initiated within individual replicons.

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