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Distinct controls of DNA replication and of nuclear division in the cell ...

Distinct controls of DNA replication and of nuclear division in the cell ...

t12m30 -2010--|00-

t12m30 -2010--|00- O-

10 15Time (h)Fig. 6. Effect of different growth rates on variation in RNAand DNA ratio during the cell cycle of Scenedesmusquadncauda. Values of the mean irradiance and dilution ratefor individual curves: curve 1, 108 Wm , 0-11 h ; curve 2,90 W m , 0-08 h ~ ' ; curve 3, 60 W m , 0-045 h2010Time (h)Fig. 5. Variation in the ratio of RNA to DNA insynchronous cultures of Scenedesmus quadncauda grown fordifferent time intervals in light. Culture conditions are thesame as described in Fig. 1A. A. Open symbols indicate thevalues found in light-grown cells and closed symbols in cellsdeprived of light. The time at which the populations wereput into the dark or illuminated again are marked by arrows.B. Deconvolution of composite curve of RNA to DNA ratio(curve 4) found in light-growing culture (see A) into threesimple curves (curves 1, 2, 3). The maxima on these curvesare assumed to signal the attainment of a critical amount ofRNA and minima the termination of a replication round.when RNA synthesis was stopped by nitrogen starvationthe number of DNA replications became correspondinglyreduced to attain this minimal value (Fig. 7). It indicatedthat the maximal amount of DNA synthesized in a cell isdictated by the amount of RNA synthesized some timebefore the replication of DNA was initiated (Figs 3, 7).Consequently, in a fast-growing culture, the commitmentto the second DNA replication round had been attainedbefore the first DNA replication round was terminated.The value of the RNA to DNA ratio then exceeded forsome time the expected double value of the minimal one(Fig. 6). The resulting curve recording variance in RNAto DNA ratio during the cell cycle is assumed to becomposed of several simple distribution curves (Fig. 5B).This assumption was confirmed by the experiment in10 15Time (h)Fig. 7. Variation in RNA and DNA content per cell incontrol, light-deprived and nitrogen-starved synchronouscultures of Scenedesmus quadncauda. Culture conditionsand labelling of light and dark period in control culture arethe same as in Fig. 1A. Times of light deprivation and ofnitrate withdrawal are marked by arrows. (•) RNA;(O) RNA, light-deprived; (3) RNA, nitrate-starved;(A) DNA; (A) DNA, light-deprived; (A) DNA, nitratestarved.which the population of cells were deprived of light just atthe times when maxima in the ratio of RNA to DNA werereached. The ensuing DNA replication reduced the ratioof RNA to DNA to the minimal value (about 10) (Figs 2,5A).If the darkened cells were again illuminated after theControls of DNA replications and mitosis 535

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