Organisms have evolved to co-ordinate their activities with the day-night cycle caused by caused by the Earth's rotation. Direct responses to light or darkness are important but, in addition, biological clocks have evolved to time biological processes. "Circadian" rhythms (from 'circa'-about, 'dies'-a day) are the result of the best-characterised of these biological clocks, which times events that occur once per day. Even in the absence of environmental time cues, circadian rhythms persist with a period close to 24 hours. The circadian clock regulates many aspects of metabolism, physiology and behaviour, in humans and many other organisms.
Sounds complicated? Try a simple tutorial on biological clocks.
The mechanism of the circadian clocks had been difficult to determine, until the identification of circadian rhythm mutants and their cognate genes in Drosophila, Neurospora and now in other organisms. Molecular and genetic studies indicate that the 24-hour period arises from a system of interconnected feedback loops that control the transcription of a small number of "clock genes". Negative feedback, together with a delay, is sufficient in principle to produce oscillations; real circadian clocks are more complex, and it remains a challenge to understand this complexity. Circadian rhythms are outwardly very similar in all species but the genes that make up the clock mechanisms are quite different (comparing animals, plants, fungi and cyanobacteria). Clocks probably evolved several times to perform very similar functions, so they are an example of "convergent evolution".
What is a clock gene? Try a tutorial on rhythms and clock mechanisms.
In plants, the circadian clock regulates about 10% of the genome (>2000 genes in Arabidopsis). The rhythmic function of these genes controls many processes, including leaf and petal movements, the opening and closing of stomatal pores, the discharge of floral fragrances and many metabolic activities, especially those associated with photosynthesis. The circadian clock also influences seasonal cycles that depend on day-length, including the regulation of flowering. This photoperiodic system appears to depend on the circadian clock to measure the duration of the day or night, thus monitoring the passage of the seasons.
Don't care about plants? You should! but you could try clock research in other organisms.
Arabidopsis thaliana, a model species for plant genetics, exhibits visible circadian rhythms in leaf movement and less obvious rhythms in the expression of many genes, notably chlorophyll a/b binding protein genes (CAB genes). The bioluminescent reporter gene, luciferase, has been used to visualise the circadian regulation of gene expression, creating a glow rhythm in transgenic plants. This rhythm can be monitored in single seedlings by low-light video imaging, which has allowed several groups to identify rare mutant plants with altered rhythms. From these mutants, the genes required to build the circadian clock in Arabidopsis have been identified: there are now over forty clock-associated genes.
Click here to see Video's of circadian-regulated leaf movements, hypocotyl elongation and gene expression in Arabidopsis and tobacco plants.
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