Initiation and termination of macronuclear DNA synthesis in Paramecium: effect of variation in macronuclear DNA content

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Abstract
The timing of both initiation and termination of DNA synthesis was examined in Paramecium cells with an unusually wide range of DNA contents. Variation in DNA content was increased through the action of a mutant gene, tamG, which interferes with macronuclear amitosis. The major conclusions are (1) timing of initiation of DNA synthesis is unaffected by G1 macronuclear DNA content and (2) the duration of the S phase of the cell cycle increases only slightly with increasing macronuclear DNA content. Since Paramecium cells with increased DNA content do not synthesize increased amounts of DNA, the present observations imply that the absolute rate of increase in DNA content remains constant, while the relative rate of DNA synthesis (rate per unit DNA) decreases as initial DNA content increases. The implications of these conclusions for regulation of macronuclear DNA content, cell mass, and cell cycle duration are discussed.