[Illustration: Figure 24 _A college chemistry major analyzing a sample of radioactive materials with an instrument known as a proportional beta counter._] CELL CYCLE AND GENE ACTION: LIFE IS THE SECRET OF DNA _Some circumstantial evidence is very strong, as when you find a trout in the milk._ Henry David Thoreau For a biologist interested in the mechanism of cell proliferation, the most important event in the life of a cell was, until very recently, cell division. As we mentioned, when a cell divides into two daughter cells, it undergoes a process called mitosis; mitosis itself is subdivided into four stages called prophase, metaphase, anaphase, and telophase. Mitosis in most cells takes less than one hour. Between one mitosis and the next, there can be an interval, from a few hours to several days in length, during which a cell is said to be in interphase. The entire period between the midpoints of two successive mitoses is called the cell cycle. Interphase Until a few years ago, we knew very little about interphase. In fact, in one classic book on histology,[8] while a description of mitosis required almost 12 pages, interphase was dismissed in less than six lines! The reason for this lack of interest was, of course, the fact that no adequate methods were available for studying metabolic activities of cells in interphase. The methods of high-resolution autoradiography and radiochemical analysis of synchronized cell populations have become available only in the past few years. We now know that metabolic activities during interphase are of primary importance in understanding the mechanism of cell division. It is, in fact, the orderly sequence of metabolic events occurring in interphase that leads from one mitosis to the next. The Cell Cycle