Where cells are at in the cell cycle?
As described in the yeast cell cycle video, with a budding yeast like S. cerevisiae, one can simply look at the bud size and tell the stage of the cell cycle. With tissue culture cells, one can often tell if they are in mitosis in the microscope, but one can’t tell where they are in interphase. A variety of methods are used. One is to incubate the cells with a pulse of radiolabeled DNA precursor (usually tritium-labeled thymidine). After the pulse, cells are fixed and covered with a photographic emulsion that is sensitive to radiation. After exposure and development, one examines the cells–only those in S phase will have labeled nuclei. More often, one now uses fluorescence-activated cell sorters (Fig. 14.4). These monitor the amount of DNA per individual cell and count cells in each phase. II. Cell cycle progression can be arrested in different cell types at certain checkpoints (e.g., START and the restriction point for yeast and animal cells, respectively, Fig. 14.5, 14.6, 14.8, 14.
