biological clock The mechanism, presumed to exist within many animals and plants, that produces regular periodic changes in behaviour or physiology. Biological clocks underlie many of the biorhythms seen in organisms (e.g. hibernation in animals). They continue to run even when conditions are kept artificially constant, but eventually drift out of step with the natural environment without the specific signals that normally keep them synchronized. Studies in the fruit fly Drosophila have revealed the molecular basis of the biological clock, and similar mechanisms are thought to occur in other animals, including mammals. It involves various proteins, some of which serve as transcription factors for their own genes, particularly PER (encoded by the per gene) and TIM (encoded by the tim gene). These form part of a negative feedback loop in which the concentration of the proteins cyclically rises and falls. The timing of each cycle is determined by the time required for transcription, export of messenger RNA to the cytoplasm, translation, and, crucially, the formation of PER–TIM dimers – the only form in which these two proteins can enter the nucleus. Also, some of the proteins, including TIM, are sensitive to light and are degraded during the day. Hence, the biological clock is entrained to the day–night cycle.

biological clock An endogenous, physiological mechanism, whose exact nature has not been determined, that keeps time independently of external events, enabling organisms to determine and to respond to daily, lunar, seasonal, and other periodicities. Its existence has been inferred from the observation of organisms which retain rhythmic activity under constant conditions. See also circadian rhythm.

biological clock An endogenous, physiological mechanism, whose exact nature has not been determined, that keeps time independently of external events, enabling organisms to determine and to respond to daily, lunar, seasonal, and other periodicities. Its existence has been inferred from the observation of organisms which retain rhythmic activity under constant conditions. See also CIRCADIAN RHYTHM.

biological clock Internal system in organisms that relates behaviour to natural rhythms. Functions, such as growth, feeding, or reproduction, coincide with certain external events, including day and night, tides, and seasons. These ‘clocks’ seem to be set by environmental conditions, but if organisms are isolated from these conditions, they still function according to the usual rhythm. If conditions change gradually, the organisms adjust their behaviour gradually.