Mitosis is a continuous process; the following stages of the process are designated only for convenience. During _interphase_ the cell is busy metabolizing, synthesizing new cellular materials, and preparing for self-duplication by synthesizing new chromosomes. In _prophase_ the chromosomes, each now composed of two identical strands called chromatids, shorten by coiling, and the nucleolus and nuclear membrane disappear. During _metaphase_ the chromosomes line up in one plane near the cell equator. At _anaphase_ the sister chromatids of each chromosome separate, and each part moves toward the ends, or poles, of the cell. During _telophase_ the chromosomes uncoil and return to invisibility; a new nucleus, nucleolus, and nuclear membrane are reconstituted at each end, and division of the cell body occurs between the new nuclei, forming the two new cells. Each daughter cell thereby receives a full set of chromosomes, and, since the genes are in the chromosomes, each daughter cell has the same genetic complement. [Illustration: Figure 7 _Photomicrograph of cells of the Trillium plant, which has five chromosomes, in anaphase. Note the duplicate sets of chromosomes moving to opposite poles of the cell._] All life processes use up energy and therefore require fuel. The mitochondria have a central role in the reactions by which the energy of sugars is supplied for cellular activity. The importance of this vital activity is obvious. In this booklet, however, we are concerned with the processes, involving nucleic acids and proteins, that can be described as making up “the gene-action system”. The gene-action system is the series of biochemical events that regulate and direct _all_ life processes by “transcription” of the genetic “information” contained in molecules of DNA. RADIOACTIVE ISOTOPES: THE BIOLOGICAL DETECTIVES _Man ... has found ways to amplify his senses ... and, with a variety of instruments and techniques, has added kinds of perception that were missing from his original endowment._ Glenn T. Seaborg Atomic Structure Practically everyone nowadays is to some extent familiar with the atomic structure of matter. Atomic energy, nuclear reactors, and radioisotopes are terms in everyday usage. However, to appreciate how radioisotopes can be applied to the study of life processes, we must have at least a working knowledge of their properties, their preparation, and their limitations. It is therefore appropriate to examine them in detail so that the succeeding chapters will be more easily understood.