Levels of Enzyme Activity and Deoxyribonucleic Acid Synthesis in Mammalian Cells Cultured from the Animal

Abstract

The replication of DNA in the cells of higher forms occurs only during part of the interphase of their division cycle (l-8).As a result, the cycles of plant and animal cells may be divided into four discrete phases: G1, the postmitotic period preceding DNA synthesis; S, the period of DNA synthesis; Gz, the premitotic period following DNA synthesis; and M, the relatively brief period of mitotic division (3).The very low rates of DNA synthesis and cell division characteristic of many tissues and organs of the adult mammal imply that control of cell multiplication involves blocks or restrictions in one or more of the phases of the division cycle.In fact, the diploid nature of most of these cells indicates that their control mechanisms are largely confined to the Gr, rather than to the Gs, period.This is consistent with the observation that the lengths of the S and Gz periods are relatively constant for most mammalian cells, whereas the G1 period generally governs the doubling time (9-l 1).Previous observations (12, 13) suggested that kidney cortex cells cultured directly from the rabbit might serve as a useful tool for the study of the chain of events of the G1 period1v2 that lead to DNA synthesis and cell division.During this period, marked changes in RNA metabolism were observed in these cells (13).For the first 12 hours of culture, the rate of turnover of nuclear RNA remained at the in a-ivo level, although the low initial rate of synthesis of stable ribosomal RNA increased progressively.Between 12 and 22 hours, a sharp increase in the turnover rate of nuclear RNA occurred, and this elevated rate was maintained until the onset of DNA synthesis.Addition of p-fluorophenylalanine or low levels of actinomycin D (0.01 pg per ml) prevented the increase in the turnover rate of nuclear RNA, and in a parallel fashion, the subsequent synthesis of DNA.Early protein and RNA synthesis, therefore, appeared to be essential for later DNA replication.Once the high rate of RNA turnover was established in the