THEORETICAL ANALYSIS OF A METHOD FOR DETERMINING THE PATTERN OF MACROMOLECULAR SYNTHESIS DURING THE CELL CYCLE

The dual-labelling centrifugal-elutriation method has been extensively used to study patterns of macromolecular synthesis and accumulation in the yeast cell cycle. Cells are long-term labelled with a radioactive precursor for about 1 • 5 cycles (a measure of macromolecular mass), then pulse-labelled with a precursor containing a different radioactive isotope for the final CM cycle (a measure of rate of synthesis). Harvested cells are fractionated into cell cycle stages by centrifugal elutriation, and changes in pulse: long-term labelling during the cycle determined. This pattern of change is com-pared with theoretical changes in rate: mass calculated for various patterns of synthesis. Using this method, it has been suggested that rates of synthesis and accumulation of several types of RNA and protein all increase exponentially through the cycle. In contrast, experiments using synchronous cultures or zonal centrifugation for cell cycle analysis have suggested other synthetic patterns, including periodic doubling in rate in each cycle. In this paper we analyse whether the dual-labelling, centrifugal-elutriation method is capable of discriminating between exponential and periodic rate-doubling patterns. Three possible sources of imprecision in the method and its application are examined. (1) Theoretical rate: mass curves have been simulated for macromolecules with a wider range o