Cell Cycle Logic
There are two requirements for successful long-term cell
proliferation. One is that the steps of the chromosome
replication–division cycle occur in a correct and fixed
order: deoxyribonucleic acid (DNA) replication (S phase)
always precedes chromosome segregation (mitosis or M
phase), followed by cell division (cytokinesis). The second
is that this sequence of chromosomal events repeats with a
period equal to the mass doubling time (the time required
to double cytoplasmic mass). The first set of constraints is
necessary to maintain the integrity of the genome; the
second set is necessary to maintain the cell’s nucleocytoplasmic
ratio within viable bounds.
The correct sequence of events is a robust characteristic
of the chromosome cycle. If an early event is blocked, then
usually none of the following events takes place, suggesting
that cell cycle events form a dependent sequence. For
example, if DNA replication is blocked by a drug or a
conditional mutation, then later events – mitosis and
cytokinesis – are not initiated. If mitosis is blocked, then
cell division and the subsequent S phase will not take place.
If cell growth is blocked by nutrient deprivation or
antigrowth signals, the cell cycle will arrest in G1 or G2
(depending on cell type)