Control of the initiation of DNA synthesis in 3T3 cells: low-molecular weight nutrients.

Sparse 3T3 cells, in excess serum, become quiescent in the early G(1) (or G(0)) phase of the cell cycle if the cells are cultured in low concentrations of amino acids, glucose, or phosphate ion. These quiescent cells then initiate DNA synthesis if the concentration of the limiting nutrient is increased. In normal medium, DNA synthesis in the same 3T3 cell line is controlled by serum factors. The results demonstrate the complexity of the control by external agents of DNA synthesis in these "normal," density-dependent cells.