This animation explores the molecular interactions that regulate circadian rhythms in Drosophila fruit flies.

Most organisms have circadian rhythms, biological processes that operate on 24-hour cycles. These rhythms are regulated by an internal timekeeping mechanism called the biological clock. The animation explores a series of cyclically expressed proteins that maintain the biological clock in Drosophila.

The animation begins by introducing the gene period (per), which encodes a protein, PER, that accumulates and degrades in regular cycles. The per gene is activated by the transcription factors CYC and CLOCK. As PER builds up during the night, it represses the CYC-CLOCK complex, creating a negative feedback loop. After daybreak, PER is degraded and CYC-CLOCK is reactivated, starting the cycle again.

The next section of the animation introduces the timeless (tim) gene, which encodes a protein, TIM, that binds to and stabilizes PER. The animation also discusses the roles of Doubletime, a protein that accelerates the degradation of PER, and Cryptochrome, a light-activated protein that destroys TIM. The animation ends by demonstrating how a mutation in Doubletime affects the rest of the biological clock pathway.

This animation is a clip from a 2000 Holiday Lecture Series, Clockwork Genes: Discoveries in Biological Time. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different proteins or parts of the pathway.