The initiation of deoxyribonucleic acid synthesis in the connective-tissue cell, with some observations on the function of the nucleolus

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Abstract
Observations were made on connective-tissue cells derived from explants of rat-heart tissue. 85 to 90% of these cells emigrate from the explants before they begin to synthesize DNA. The lag period preceding the onset of synthesis of DNA lasts about 20 hr.; for 12 hr. of this phase, the cells are outside the explant. During the lag period there is continuous synthesis of both protein and ribonucleic acid. These two processes are prerequisites for the synthesis of DNA: if either is interfered with this latter synthesis does not take place. The duration of synthesis of DNA varies widely. A median figure, obtained from several hundred cells, puts the beginning of this phase between 13 and 14 hr. before metaphase and its end between 3 and 4 hr. before metaphase; mitosis is thus immediately preceded by a period in which no DNA synthesis occurs. An analysis of the patterns of nuclear and nucleolar labelling in radioautographs of cells exposed to tritiated thymidine indicates that DNA synthesis is initiated at the nucleolus.