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. Author manuscript; available in PMC: 2021 May 1.
Published in final edited form as: Gastroenterology. 2020 Feb 13;158(7):1948–1966.e1. doi: 10.1053/j.gastro.2020.01.050

Figure 2: Overview of Circadian Clock Machinery.

Figure 2:

Humans/primates are diurnal animals with the feeding/active period occurs during the light hours. However, mice (and most rodents) are nocturnal, with the feeding/active period occurring in the dark hours. The cellular molecular clock is a series of transcription-translation feedback loops that include the transcriptional acti vators (CLOCK and BMAL1; the positive limb) and repressors (PER and CRY; the negative limb). The PER:CRY complex repress their own activators CLOCK:BMAL1. CLOCK:BMAL1 also promote REV-ERBs and RORs which add another layer of regulation over the circadian clock and regulate hundreds of additional genes.