Oscillatory Mechanisms Underlying the Drosophila Circadian Clock

This animation depicts a schematic of the Drosophila circadian clock mechanism in a single, pacemaking lateral brain neuron. Circadian clocks allow organisms to exhibit rhythmic behaviors and processes based on a nearly 24-hour cycle in the absence of any light stimulation. In the fruit fly, the basic molecular mechanism consists of two intertwined transcription-translation negative feedback loops. One loop--the "positive loop"--controls the rhythmic expression of a positive transcription factor gene, Clock . The second loop--the "negative loop"--controls the transcription of period and timeless , two genes that encode repressor proteins. The loops are intertwined because Period and Timeless repress transcription mediated by the transcription factors Clock and Cycle, whereas the Clock:Cycle heterodimer drives transcription of period and timeless , as well as that of vrille , a repressor of Clock expression. Light cycles can synchronize the Drosophila circadian pathway by directly stimulating cryptochrome -dependent degradation of Timeless.