The time of the cell cycle

Living cells reproduce by duplicating their content and dividing in two. At a minimum, the cell cycle comprises the set of processes that a cell must go through in order to faithfully replicate its DNA and to segregate duplic ated chromosomes (mitosis) in two separate daughter cells. Recent data sugg est that a central system operates, which is distinct from the machinery pe rforming the essential processes of the mitotic cell cycle. This central cl ock is regulated at specific checkpoints where feedback signals from the do wnstream processes can delay the triggering of the next step before the pre vious one is achieved. This biochemical device of cell cycle control is bas ed on the regulation of the activity of cyclin-dependent protein kinases (c dks). Cyclins bind to cdks and regulate their ability to phosphorylate spec ific substrates. Moreover, the activity of some cdks, and therefore the abi lity of a given cell to divide, seems to depend on its "history". This info rmation could play a crucial role in various processes such as differenciat ion, organogenesis and senescence.