Circadian clocks ensure that we modulate our physiological behaviors, like sleeping and eating, in a rhythmic, twenty-four hour cycle. They are present in all living cells, from cyanobacteria to humans. The circadian clock is temperature compensated, meaning that it does not speed up or slow down regardless of temperature; it's also entrained, meaning that it can reset according to external cues, like light, temperature and food. The selective advantage of entrainment becomes apparent you debark from a flight that crosses time zones.
On a molecular level, this clock appears to be based on a feedback loop whereby the products of a set of clock genes negatively regulate their own expression, guaranteeing a rhythmic production. The specific clock genes are not conserved across distant species, but the underlying mechanism is the same.
Yet John O’Neill and Akhilesh Reddy at the University of Cambridge have used two very clever experimental systems to show that a circadian rhythm can persist even when these genes are not transcribed into RNA. The first system they used is human red blood cells, which lack a nucleus, so they have no transcriptional machinery or even genes to transcribe. The second is the single-celled alga Osteococcus tauri, one of the most primitive nucleated cells known.
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