When a clock gene is switched on, it directs the cell to produce proteins, which eventually trigger a signal that switches the gene off.

BBC: Body clock 'set before birth'

This is because of molecular switches that turn parts of the genome on and off depending on what the cell in question is up to.

ECONOMIST: Human genomics

That you need a profusion of such switches to get the right pattern of genes turned on and off in a given cell at a given time is obvious.

ECONOMIST: Human genomics