Waking, and Blooming, in Rhythm

What are circadian rhythms?These timing systems dictate when plants will bloom, force people to fall asleep at their desks, urge birds to fly south, and influence a host of other activities. While circadian rhythms run on a 24-hour clock, others also exist, including tidal, lunar, and annual rhythms.Which organisms have them?A lot; from bread molds to humans. Well-studied rhythms include those in cyanobacteria, the bread mold Neurospora crassa, rice, Arabidopsis, fruit flies, mice, Syrian hamste

Maria Anderson
Feb 15, 2004

What are circadian rhythms?

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These timing systems dictate when plants will bloom, force people to fall asleep at their desks, urge birds to fly south, and influence a host of other activities. While circadian rhythms run on a 24-hour clock, others also exist, including tidal, lunar, and annual rhythms.

Which organisms have them?

A lot; from bread molds to humans. Well-studied rhythms include those in cyanobacteria, the bread mold Neurospora crassa, rice, Arabidopsis, fruit flies, mice, Syrian hamsters, and humans. Some researchers say that the mechanisms found in rice and Arabidopsis might be the same ones found in all plant species. The clock components found in mice and humans also might be universal for all mammals. While all organisms share the same basic mechanism, the individual components differ, suggesting that clocks evolved independently.

What regulates circadian rhythms?

Light sets and resets them, but temperature plays a role, too. Cryptochromes, blue-light detectors found in almost all organisms, transmit signals to circadian genes and proteins, cumulatively known as the central oscillator, which keep the clock in sync. The oscillator's key feature, a negative-feedback loop in transcription, is well described in Drosophila and mice but unclear in Arabidopsis. Mammals detect light through their eyes, and their oscillator components are centralized in the suprachiasmatic nucleus in the hypothalamus. Plants, however, rely heavily on light for energy, so they have light receptors throughout their structures.

What behaviors do the clocks control?

A wide range, from binary fission in bacteria to sleep/wake cycles in humans. Circadian rhythms regulate when nitrogen is fixed by cyanobacteria; when Neurospora disperses its conidia, or asexual reproductive spores; when birds migrate; and when some mammals hibernate. In plants, circadian-driven processes include hypocotyl elongation, seasonal flowering, photosynthesis, leaf movements, gas assimilation, and carbon and sulfur metabolism.

What happens if the clock goes awry?

Arrhythmicity in plants, which occurs if clock genes mutate or external stimuli send mixed signals, can cause early flowering. In humans, hormonal fluctuations caused by aging alter the amplitudes of their circadian rhythms, disrupting sleep and moods. One disorder, called advanced sleep-phase syndrome, resets the clock four or five hours faster than normal, forcing people to fall asleep in early evening and to awaken in the predawn hours.

- Maria W. Anderson