Cell Division Control: Why Clocks Are Inadequate

Abstract

The cell division cycle and circadian rhythms have one obvious property in common: both comprise an event or series of events which is repeated at regular intervals. The course of the cell cycle is similar in nearly all eukaryotic cells. Cells at birth enter the G1 phase, which leads up to the S phase during which the nuclear DNA and other chromosomal components are replicated. S phase is followed by the G2 period, at the end of which the chromosomes are separated and segregated into daughter nuclei during mitosis. Cell division normally follows shortly after mitosis, and the two daughter cells embark on their cycles. The events of the cell cycle and their temporal organisation are broadly similar across a range of cell types. In contrast, many diverse biological processes show the periodic pattern of behaviour covered by the umbrella term “circadian rhythm”. Processes which behave in this way include cellular events such as photosynthetic activity, sporulation in some fungi, and even cell division; whole organism behaviour of many types in higher animals and plants; and events such as pupal eclosion in Drosophila, which show a circadian rhythm across a population, though of course each individual only undergoes the process on one occasion.