It is not very fruitful to discuss whether proteins or nucleic acids are more important. That question is something like the one about the chicken and the egg. We cannot think of one without thinking of the other. Although our insight into the mutual dependence of these two materials has greatly increased in recent years and although we know the relation between them is a fundamental factor in such events as reproduction, mutation, and differentiation (or specialization) of cells, our understanding of their interplay is far from complete. Real understanding of the relation between them would give us insight into the essence of growth—both normal and abnormal—or, indeed, one could almost say, into the complexity of life itself. [Illustration: Figure 5 _Photomicrograph of Paramecia, one-celled animals, magnified 1100 times. Many of the same structures that appear in Figure 3 can be seen here. This photo was taken with an “interference” microscope designed to permit continuous variation of contrast in the subject under study._] Practically all the DNA of most cells is concentrated in the nucleus. RNA, on the other hand, is distributed throughout the cell. Some RNA is present in the nucleus, but most of it is associated with minute particles in the cytoplasm known as microsomes, some of which are especially rich in RNA and are accordingly named ribosomes. These are much smaller particles than the mitochondria. [Illustration: Figure 6 _Stages of the mitotic cycle in a hypothetical cell with four chromosomes._] (a) Interphase (b) Early prophase (c) Prophase (d) Metaphase (e) Anaphase (f) Telophase (g) Interphase, daughter cells Mitosis One of the most remarkable characteristics of cells is their ability to grow and divide. New cells come from preexisting cells. When a cell reaches a certain stage in its life, it divides into two parts. These parts, after another period of growth, can in turn divide. In this way plants and animals grow to their normal size and injured tissues are repaired. Cell division occurs when some of the contents of the cell have been doubled by replication, or copying (to be discussed later). The division of a cell results in two roughly equal new parts, the daughter cells. The process of cell division is known as mitosis and is diagrammed in Figure 6.