A. General Features The interval between the birth of a diploid cell and its subsequent division into two

daughter diploid cells is known as the cell division cycle, the mitotic cycle, or more simply, the cell cycle. The cell cycle is the life history of a dividing cell, comprising the duplication of its components increases in cell and nuclear volume, changes in chromatin conformation, and temporal changes in gene expression and enzyme activity which enable the cell to perform its many functions. In a now classic paper Quastler and Sherman1 used labeling procedures to measure the duration of the cell cycle and its component phases. The technique consists of pulse-labeling tissues with methyl-3H-thymidine ([3H]-TdR), followed by monitoring the rhythmic flow of a labeled cohort of cells into and out of mitosis. The relationship between the percentage of labeled mitoses (PLM) and time following the start of labeling can enable estimates of the duration of the cell cycle, mitosis, postmitotic interphase (Gl), DNA synthetic-(S)-phase, and postsynthetic interphase (G2) for a given meristem. For detailed accounts of the methods of calculations of these parameters see either Mitchison2 or Gould.3