Scientific Back ground Discoveries of Molecular Mechanisms Controlling the Circadian Rhythm

Observations that Organisms adapt their physiology and behavior to the time of the day in a circadian fashion have been documented for a long time and are commonly agreed to have begun with the observation of leaf and flower movements in plants. For example, the leaves of mimosa plants close at night and open during the day. In 1729, the French astronomer Jean Jacques d’Ortous de Mairan placed a mimosa plant in the dark and observed that the leaves still opened and closed rhythmically at the appropriate time of the day, suggesting an endogenous origin of the daily rhythm (Figure 1). About two hundred years later, the German plant physiologist and pioneer of circadian rhythm research, Erwing Bünning, painstakingly connected the leaves of a bean plant to a kymograph and recorded the movements of the leaves during normal day/night

cycles and under constant light conditions. He observed that the rhythm of leaf movement persisted. The question of whether circadian behaviors in plants and animals were governed by an endogenous clock, or were a mere reaction to external stimuli of a circadian nature, would be hotly debated for decades. Eventually, the existence of an endogenous circadian clock would finally become established well into the 20th century.

Scientific Background Discoveries of Molecular Mechanisms Controlling the Circadian Rhythm.

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