Question

What happens when the s phase cannot enter the g0 phase?

Answer 1

The importance of the G0 phase is that during cell division when the cell get a signal that there are enough of cells (not more required) or there is a mutation in the cell that need to be fixed before they differentiate fully or the cell during its division gets damage then it is signalled to rest, and it is this phase that cell leave the cycle and quit dividing. It could be temporary resting period (so when cell again required, or damaged got fixed) then the cells will again join the cycle and continue to divide again or it can also be permanent for example neurons, which are resting in G0 phase and never divides again.