Figure 1.

Transcriptional feedback loop constituting molecular biological clocks. The central clock of the SCN, driven by light, synchronizes the clocks of peripheral tissues, including the liver. At the heart of the molecular complex are the clock proteins CLOCK and BMAL1 that heterodimerize to induce transcription of the PER and CRY genes. This feedback loop takes approximately 24 h to complete and is the molecular basis of the mammalian biological clock. BMAL1: Brain and Muscle Arnt-like protein-1; CLOCK: Circadian Locomotor Output Cycles Kaput; CRY: Cryptochrome; PER: Period; SCN: Suprachiasmatic Nucleus.