Biology
Molecular mechanisms of circadian rhythms in nocturnal rodents
Quick fact
In nocturnal rodents, the core clock genes CLOCK and BMAL1 activate transcription of Per and Cry genes, whose own proteins then inhibit CLOCK and BMAL1, forming a feedback loop that takes about 24 hours to complete.
Why this is interesting
Picture a mouse that runs on its wheel at the same time every night, even in constant darkness. How does its body know when it's time?
Read the full explanation
Understanding Molecular mechanisms of circadian rhythms in nocturnal rodents
Imagine a daily biological timer that runs on a 24-hour schedule. In nocturnal rodents like mice, this timer is built inside nearly every cell. The engine is a pair of 'activator' proteins, CLOCK and BMAL1, which bind to DNA and switch on downstream genes. Among those switched-on genes are the 'repressor' genes Per and Cry. The Per and Cry proteins accumulate, and when enough of them are present, they shut off CLOCK and BMAL1, halting their own production. Over time, the repressors are degraded, and the cycle starts again. Each cycle takes about 24 hours, creating a self-sustaining rhythm. This loop is fine-tuned by the master clock in the brain's suprachiasmatic nucleus (SCN), which receives light signals to stay synchronized with the environment.
A deeper explanation
The core mechanism is a transcription-translation negative feedback loop. In the cell nucleus, CLOCK and BMAL1 heterodimerize and bind to E-box enhancer sequences in the promoters of Per1/Per2 and Cry1/Cry2, driving their transcription into mRNA. These mRNAs are translated in the cytoplasm into PER and CRY proteins, which form complexes and translocate back into the nucleus. There, they inhibit the transcriptional activity of CLOCK/BMAL1, reducing their own transcription. The timing of the loop is set by post-translational modifications: casein kinase 1 (CK1δ/ε) phosphorylates PER, targeting it for degradation, which delays the repressor's accumulation. The degradation of PER and CRY eventually relieves the inhibition, allowing the cycle to restart. This entire cycle takes about 24 hours, generating rhythmic expression of many output genes. Light input from the eyes reaches the SCN via the retinohypothalamic tract, and light-induced depolarization triggers CREB-mediated transcription of Per genes, which shifts the phase of the clock, enabling entrainment to the day-night cycle. This molecular clock drives daily rhythms in activity, body temperature, and hormone secretion.