Molecular clocks - PERpetuating the PASt

Many periodic physiological events — ranging from photosynthesis in plants to breeding cycles in animals — are controlled by an endogenous molecular pacemaker. In the fruitfly, this system is known to be controlled by, among others, the product of theperiod gene (per). The per gene has now been cloned in mice and humans. Not only does it show striking homology to fruitfly per but, in humans, expression of Perprotein oscillates in the centre of the brain in which the molecular clock is thought to be found.