Vegetative C. reinhardtii cells grow in a long G1 phase followed by n alternating rounds of S and M phases terminated by cell division into 2n daughter cells. The alternating rounds of S/M phases occur very rapidly, without G1 or G2 phases, and are punctuated by cell divisions preventing any multin-uclear division intermediates (Lien and Knutsen 1976, 1979; Coleman 1982; Craigie and Cavalier-Smith 1982; Donnan and John 1983). Under physiological conditions of alternating light and dark periods the cell cycle becomes synchronized so that growth occurs during the light phase and cell division occurs during the dark phase (Lien and Knutsen 1976, 1979).
It was proposed that the C. reinhardtii cell cycle is governed by a combination of a "sizer" and "timer" (Donnan and John 1983, 1984), which was recently confirmed by observation of a single cell (Matsumura et al. 2003). The sizer measures attainment of a commitment size (Spudich and Sager 1980; Donnan and John 1983, 1984; Umen and Goodenough 2001) (Fig. 2), which is followed by a postcommitment period of more or less fixed duration (measured by a timer) preceding the beginning of the S/M cycles (Donnan and John 1983, 1984; Umen and Goodenough 2001; Matsumura et al. 2003). The sizer determines not only when the cells commit but also how many times they will divide. Cells that have passed commitment will complete one round of S phase and mitosis even in the absence of nutrients and exogenous energy (Donnan and John 1983, 1984). On the contrary, cells that have not passed commitment when growth is stopped by withdrawal of light or nutrients do not divide and stay in G1 phase until resupplied. If the light supply continues after the attaining of the first commitment point, cells can attain additional commitment points, each of them allowing one round of DNA replication and mitosis (Fig. 2).
After passing commitment, cells remain in a G1-like phase, the post-commitment period, which lasts for another ~5-8 h and precedes the initiation of the first S/M phase. There is no obvious difference between the pre- and postcommitment period and both of them are therefore usually designated as G1. However, committed cells require neither growth nor energy to divide so the two periods strictly differ physiologically. The postcom-mitment period is an equivalent of late G1 phase; late G1 phase is the period when the origins of replication become licensed for replication and S phase CDKs become activated (Nasmyth 1996). It has been proposed to call this phase pre-S (pre-synthetic) in algal cell cycles (Zachleder et al. 1997).
Attaining commitment is regulated by growth conditions: higher amounts of light or a longer light period allow more commitment points to be attained (Zachleder et al. 1997). Interestingly, attainment of commitment can also be modulated by light properties. Blue light delays the attainment of commitment leading to a larger cell size at commitment followed by a delay in the cell division and division into more cells (Munzner and Voigt 1992; Olden-hof et al. 2004a,b, 2006), while under red light cells attain commitment faster and divide sooner into fewer cells (Oldenhof et al. 2004a,b, 2006). The attainment of commitment point/s and cell division probably involves active CDKs because CDK activity is correlated with the increase in the amount of p34 protein and its phosphorylation (John et al. 1989) and also with the increase in CDK-like histone H1 kinase activity (Zachleder et al. 1997).
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