Transcripts from the Circadian Clock: Telling Time and Season

We all know it when we wake mere moments before an alarm clock is scheduled to wake us: our body clock made the alarm clock redundant. This phenomenon is driven by an endogenous timer known as the biological, or circadian clock. Each revolution of the Earth about its own axis produces periods of light and dark which define what we all experience as a ‘day’. This profound cyclic variation in solar energy is responsible for driving the evolution of adaptive responses as early as 3.8 billion years ago (Ditty et al., 2003), as identified in the ancient freshwater cyanobacterium Synechococcus elongatus (Kondo et al., 1993). Since this time the circadian clock appears to have arisen independently in prokaryota and eukaryota (Young and Kay, 2001) underscoring the importance of the circadian clock for reproductive fitness under daily rhythmic environments (Woelfle et al., 2004; Johnson et al., 2008). In theory, a time keeper might have been needed to temporally segregate incompatible processes such as nitrogen fixation and photosynthesis which both occur in the same cell, e.g. in Oscillatoria (Stal and Krumbein, 1987). Or to hide cellular processes vulnerable to disruption by solar (i.e. UV) radiation like DNA replication in the night- to ‘escape from light’ (Darwin, 1868; Pittendrigh, 1993; Crosthwaite et al., 1997). Whatever the reason, their ubiquity within cells and across taxa suggests a clear advantage for life able to anticipate and coordinate internal biological processes to the daily light-dark cycle. ‘Circadian’ derives from the Latin circa dies, roughly translating to, ‘approximately a day’. Careful consideration was given creating this term (Halberg et al., 1959): since daily endogenous rhythms are typically precise to the minute (Richter, 1968), but rarely exactly 24 hours. More often the internal or ‘subjective day’ of an organism is slightly faster or slower than 24 hours. In essence, circadian rhythms have been defined thus (Pittendrigh, 1960): 1. repeat once a day, 2. persist in the absence of environmental timing cues, 3. persist irrespective of temperature or temperature fluctuations, and 4. synchronize to local environmental timing cues.

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