Circadian Regulation of Cell Division 7
Fig. 1 Circadian gating of cell division in different cell types. InChlamydomonasor os-
teoblasts the clock gates cell division at the G1/S transition. In contrast, in hepatocytes,
a circadian regulation of cell division is observed at the G2/M transition through the reg-
ulation of the CDK inhibitorWEE1.InEuglena, the clock can arrest the CDC at all major
transitions
pears to gate the CDC at various stages of cell cycle progression, including
G1/S, S/G2 and G2/M (Fig. 1).
Finally, it should be mentioned that while the molecular link between the
circadian clock and the CDC has been characterized in cells re-entering or
exiting the CDC and upon checkpoint activation, the molecular basis of cir-
cadian gating of cell division in cycling cells, remains to be established.
4
What about Plants?
Direct evidences for a circadian regulation of cell division in higher plants
are still lacking. However, based on the ubiquitous nature of circadian reg-
ulation of cell division in multicellular organisms, the essential role of the
circadian clock in regulating plant physiology, it is likely that such a con-
trol will also exist in plants. On the other hand “green” organisms such as
Chlamydomonas, which belong to the green lineage, are potentially valuable