The GO-phase and commitment to cycle

Although cells in culture and cells still in the body have similar durations for S, G2 and M, it is apparent that variations in the 

Duration (h) of cell cycle phases in cultured cells 

Results have been pooled from several sources (Firket, 1965 Cleaver, 1967 Lipkin, 1971 Puck, 1972) to give a general range of data which should be compared to the results for human amnion cells (Sisken and Morasca, 1965) and mouse L5178Y cells (Defendi and Manson, 1963). As the duration of mitosis is short and not always reported, where known it has been shared between /Gl and tG2. T is the total cell generation time /Gl, /S and /G2 are the durations of the Gl, S and G2-phases, respectively. (See Fig. 10.1 for a diagrammatic representation of the cell cycle.) 

Cells which have spent a long time in Gl lose some of the enzymes typically present in dividing cells — particularly those concerned with DNA synthesis. These cells have traditionally been said to be out of cycle or in GO. The implication of the GO-phase is that to leave GO-cells require a stimulus to urge them past a barrier and back into cycle. Pardee (1974) has suggested that whenever cells are exposed to suboptimal physiological conditions they enter a quiescent phase, and that there is a single restriction point in Gl which regulates their re-entry into a new round of the cell cycle. 

There are many experimental conditions where cells are put into to or taken out of GO in order to study the changes associated with onset of proliferation. The following examples are referred to elsewhere in this book. 

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