[Molecular mechanisms of the biological clock]

Considerable progress has been made in elucidating the mechanisms underlying the generation of circadian rhythmicity. This review describes recent observations and hypothesis on the nature of circadian biological clock in different organisms, such as cyanobacteria (Synechococcus; in which the first noneukaryotic clock was described), Neurospora and Drosophila (representing respectively a fungal and insect system of crucial value for the molecular dissection of circadian timing systems), as well as mammals. Being specific for particular group of organisms, a common model of the circadian biological clock can be depicted which involves a negative transcription/translation-based feedback loop, containing positive and negative elements, and clock genes, as well as clock controlled genes that are directly related to rhythmic metabolism and behavior.