ABSTRACT
Cristofalo et al. provided evidence that representatives from three classes of growth factors must be present to completely stimulate resting cells to enter the G1 phase of the cell cycle. The concept of two distinct growth blocks (designated as the G0 and late G1 blocks by E. Wang) should be useful in understanding the differences between quiescent and senescent cells. Whereas only growth factors are necessary to induce cells arrested at the first block to proliferate, transformation of cells (e.g., by SV40 virus) is required to induce senescent cells to proliferate. Finally, the DNA sequence itself may play important roles. Howard has proposed that repetitive sequences in DNA, when unmasked, may be binding sites for regulatory proteins needed elsewhere for specific functions such as DNA replication. The proteins and the DNA must be assembled into a functional replication complex, represented in the cartoon by the small solid circle “traveling” around the cell cycle.
