description
Circadian rhythm is an internal biological clock, which enables to sustain an approximately 24-hour rhythm in the absence of environmental cues. In mammals, the circadian clock mechanism consists of cell-autonomous transcription-translation feedback loops that drive rhythmic, 24-hour expression patterns of core clock components. The first negative feedback loop is a rhythmic transcription of period genes (PER1, PER2, and PER3) and chryptochrome genes (CRY1 and CRY2). PER and CRY proteins form a heterodimer, which acts on the CLOCK/BMAL1 heterodimer to repress its own transcription. PER and CRY proteins are phosphorylated by casein kinase epsilon (CKIepsilon), which leads to degradation and restarting of the cycle. The second loop is a positive feedback loop driven by the CLOCK/BMAL1 heterodimer, which initiates transcription of target genes containing E-box cis-regulatory enhancer sequences

external resources
NCBI:83084
KEGG:hsa04710
PUBMED:17586087
PUBMED:12507418
PUBMED:10864977
PUBMED:18802415
PUBMED:17463251
PUBMED:12150932
PUBMED:11687489
PUBMED:12724733
PUBMED:12015613
PUBMED:11533719
PUBMED:12397359
PUBMED:15917222
PUBMED:11165242

genes
ARNTL , CREB1 , CRY1 , CRY2 , CSNK1D , CSNK1E , NPAS2 , PER1 , PRKAA1 , PRKAA2 , PRKAB1 , PRKAB2 , PRKAG1 , RORA , RORB , RORC , SKP1 , CUL1 , BHLHE40 , PER3 , PER2 , BTRC , NR1D1 , CLOCK , RBX1 , FBXW11 , FBXL3 , PRKAG2 , PRKAG3 , BHLHE41 ,