|MadSci Network: Zoology|
Most behaviors and physiological processes that occur on a periodic basis (such as singing in the morning or sleeping and waking) and appear to have some connection to light/dark cycles are called circadian rhythms. Usually the connection between light/dark cycles and the behavior/process is fairly loose but alteration of the light/dark cycle will result in a significant change in the periodicity of the behavior/process. These cycles are governed by a set of genes which have a periodic pattern to their expression. Usually, the presence of light will turn on one gene, which in turn activates several other genes (this type of regulation is known as downstream activation). For the gene activity to be periodic one of the downstream genes must stop the inital, upstream gene from being expressed. In the case of the genes that govern circadian rhythms in flies (and probably mammals and birds) the gene that starts the cycle encodes a transcription factor (a protein that binds to DNA and allows transcription to begin). This transcription factor can only bind DNA if it is complexed with its specific partner, forming a dimer (when the two parts are different proteins they form a heterodimer and when the parts are the same protein a homodimer is formed). To block transcription of the initial gene the downstream gene encodes a protein that binds the initial transcription factor instead of the normal dimerization partner. This stops the initial transcription factor from binding DNA and the downstream cascade of gene expression is stopped. As this also means the transcription of the competing protein is stopped then, once the population of the competing protein falls below a certain concentration, expression of the initial transcription factor can begin again. It is the cyclic nature of the expression of these genes that is the root cause of most periodic behaviors, which would include singing in the morning. So the male bird "knows" when to start singing because his genes have told him it's time to sing.
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