[The biological clock: circadian organization of the cell (author's transl)].

Unicellular systems, and particularly individual cells, are advantageous in investigating the molecular mechanism of the circadian rhythm. The greeen alga Acetabularia has proven to be a uniquely suitable organism. Experiments are described which indicate that the circadian rhythm is due to a sequence of events which are casually interrelated. A model is presented which is based on a two-step mechanism, the first step of which is the synthesis of "essential polypeptides" on 80 S ribosomes. In a second step these polypeptides are consumed. The function of the "essential polypeptides" is to change a membrane component of the circadian clock. The membrane participates in a feed-back mechanism which produces a periodic alteration of the rate of synthesis of "essential polypeptides" and of photosynthesis. It is possible to draw parallels between this circadian rhythm model and other repetitive and non-repetitive types of temporal organization within the cell, e.g., the cell cycle; however, it must be stressed that in the case of the circadian rhythm, much stronger and more efficient mechanisms exist to keep the period constant. This is true particularly for temperature compensation. Such a conservatism is understandable if one recalls that during evolution, the circadian rhythm represents an adaptive advantage which permits the organism to adjust it activities in advance to periodic processes resulting from the earth's rotation.