Cell Division Cycles and Circadian Rhythms

Abstract

Cell division cycles and circadian clocks are major periodic processes in living organisms. Circadian rhythms of cell division have been found in many eukaryotes and some prokaryotes. Circadian clocks gate cell division within discrete time windows. Among bacteria, circadian clocks have been found only in cyanobacteria. The freshwater unicellular cyanobacterium Synechococcus elon-gatusM PCC 7942, which utilizes light as an energy source, grows and divides in the daytime and stops dividing during the night. When the cells are placed in continuous light conditions, the rhythmic occurrence of cell division continues with a period of 24 h. Whether the cyanobacterial cells are rapidly growing or halted in their division cycle, the circadian clock appears to tick steadily and operates normally. This phenomenon implies an independence of circadian timekeeping from the cell division cycle. The mechanisms and significance of circadian control of the cell division cycle in cyanobacteria are discussed here.