Cell Cycle Clocks
The cell cycle, or perhaps more precisely, the cell division cycle (CDC), typically comprises a series of recurrent, relatively discrete, morphological and biochemical events (Fig. 3.1), although the specific elements may vary among different systems (Fig. 3.2). Ordinarily the CDC is loosely considered to begin with the completion of one cell division and end with the completion of the next; the time taken for one such cycle is termed generation time (g). Nesting within its time domain are the ultrafast, the metabolic, and the epigenetic domains (Lloyd et al., 1982b; see Fig. 1.1). Progress through the CDC is usually monitored by observing the overt processes of DNA replication (synthetic, or S, period) and cell division (D; or mitotic, M, period). Thus, the CDC has been divided for more than 30 years (Howard and Pelc, 1953) into four consecutive intervals—G1, S, G2, and M—where G1 and G2, respectively, designate the time gaps between the completion of division and the onset of DNA replication and between the end of replication and the onset of mitosis. We now know that both G1 and G2 (as well as S and M) are characterized by many specific, additional events, such as bud emergence (in budding yeast), chloroplast replication (in plant cells), nuclear division, or the expression of a particular enzyme, and that the CDC perhaps can be still further subdivided. As a consequence, a veritable alphabet soup of cell cycle states has emerged, replete with A, B, Go, and Gq in addition to G1, G2, M, and S [see books by Mitchison (1971), Prescott (1976), John (1981) and Lloyd et al. (1982b) for general treatments of the subject].
KeywordsCircadian Rhythm Circadian Clock Nuclear Division Cell Division Cycle Circadian Time
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