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The circadian—or biological—clock is an intrinsic, timekeeping, molecular mechanism that allows plants to coordinate physiological activities over 24-hour cycles called circadian rhythms. Photoperiodism is a collective term for the biological responses of plants to variations in the relative lengths of dark and light periods. The period of light-exposure is called the photoperiod.
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The biological clock is involved in many aspects of regulating complex physiology in all animals. It was in 1935 when German zoologists, Hans Kalmus and Erwin Bünning, discovered the existence of circadian rhythm in Drosophila melanogaster. However, the internal molecular mechanisms behind the circadian clock remained a mystery until 1984, when Jeffrey C. Hall, Michael Rosbash, and Michael W. Young discovered the expression of the Per gene oscillating over a 24-hour cycle. In subsequent years,...
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Related Experiment Video

Updated: Jul 19, 2026

Monitoring Cell-autonomous Circadian Clock Rhythms of Gene Expression Using Luciferase Bioluminescence Reporters
10:38

Monitoring Cell-autonomous Circadian Clock Rhythms of Gene Expression Using Luciferase Bioluminescence Reporters

Published on: September 27, 2012

A CLOCK-less clock.

Gad Asher1, Ueli Schibler

  • 1Department of Molecular Biology, Sciences III, University of Geneva, 30 Quai Ernest Ansermet, CH-1211 Geneva-4, Switzerland.

Trends in Cell Biology
|September 26, 2006
PubMed
Summary

The CLOCK protein is not essential for circadian rhythms in mammals. Experiments with knockout mice indicate other proteins can replace CLOCK

Area of Science:

  • Circadian Biology
  • Molecular Physiology
  • Genetics

Background:

  • Mammalian circadian rhythms rely on complex transcriptional feedback loops.
  • The PAS domain helix-loop-helix protein CLOCK was thought to be crucial in these loops.
  • A dominant-negative mutation in CLOCK supported its key role.

Purpose of the Study:

  • To investigate the essentiality of the CLOCK protein in mammalian circadian timing.
  • To determine if alternative proteins can compensate for CLOCK function.

Main Methods:

  • Analysis of Clock knockout mice.
  • Assessment of rhythmic gene expression.
  • Observation of behavioral rhythms.

Main Results:

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Rapid Analysis of Circadian Phenotypes in Arabidopsis Protoplasts Transfected with a Luminescent Clock Reporter

Published on: September 17, 2016

Related Experiment Videos

Last Updated: Jul 19, 2026

Monitoring Cell-autonomous Circadian Clock Rhythms of Gene Expression Using Luciferase Bioluminescence Reporters
10:38

Monitoring Cell-autonomous Circadian Clock Rhythms of Gene Expression Using Luciferase Bioluminescence Reporters

Published on: September 27, 2012

Parallel Measurement of Circadian Clock Gene Expression and Hormone Secretion in Human Primary Cell Cultures
06:53

Parallel Measurement of Circadian Clock Gene Expression and Hormone Secretion in Human Primary Cell Cultures

Published on: November 11, 2016

Rapid Analysis of Circadian Phenotypes in Arabidopsis Protoplasts Transfected with a Luminescent Clock Reporter
07:42

Rapid Analysis of Circadian Phenotypes in Arabidopsis Protoplasts Transfected with a Luminescent Clock Reporter

Published on: September 17, 2016

  • CLOCK protein is dispensable for rhythmic gene expression in mice.
  • CLOCK protein is dispensable for circadian behavior in mice.
  • Evidence suggests other proteins can substitute for CLOCK function.

Conclusions:

  • The CLOCK protein is not strictly required for the core circadian clock mechanism.
  • Redundancy in the circadian system allows for compensation of CLOCK function.
  • Further research is needed to identify substitute proteins and their roles.