Visualising the molecular clock

An image of a single cell from a zebrafish showing the endogenous rhythmic expression from the Period1 (per1) gene. The green, yellow and red areas show high levels of activity of a luciferase reporter gene attached to the per1 gene promoter. The per1 gene is part of an inherent molecular network that maintains the circadian rhythm in individual cells although the actual length of these cycles will gradually change over time. The light-dark cycle synchronises these endogenous rhythms of the clock to precisely 24 hours. In the transparent zebrafish, all cells are directly responsive to light, whereas in mammals, the cells are dependent on signals from a part of the brain called the suprachiasmatic nucleus (SCN), which has a direct input from the retina, to synchronise rhythms to the light-dark environment.