Since the chromosomes are duplicated and separated in the process of cell division, it is obvious that the major macromolecules at some time in the life cycle of a cell. In addition, the original cell size cannot be attained without the synthesis of RNA and cytoplasm protein, to say nothing of less complex molecules.
Since we discussed the descriptive phases of mitosis earlier, let us now attempt to understand something of the intracellular synthesis which occurs during the cycle.
The time relationships involved are highly variable in different organisms and in different cell types of the same organism. Furthermore, variations in the environmental circumstances under which cells are growing have an important bearing upon length of the cycle.
However, we can make some fairly accurate generalizations with regard to the different periods of the cycle, and as we shall we, these generalizations are related to the synthetic activity which is taking place. Experiments involving radioactively labelled precursors of DNA, RNA, and protein have provided a rather clear picture of the points in the cycle ac which these macromolecules that are so important in the control cellular metabolism are synthesized.
The total quantity of DNA is doubled during the S period, which provided a full complement of DNA for the prospective cells that will result from the next active division. The synthesis of RNA occurs between late telophase and early prophase.
Protein synthesis occurs throughout the entire cycle, although it occurs at a reduced rate during the stages of active mitosis when the chromosomes are in a state of maximum contraction.
In the light of what we know about the control of protein synthesis this indicates that enough messenger RNA is synthesized in early prophase to permit protein synthesis to continue through the other stages until late telophase, at which point more messenger RNA is template by DNA. In summary, intracellular growth occurs whenever synthetic processes result in a greater amount of protoplasm within a given cell. This ordinarily leads to cell division and with further synthesis and subsequent division, the overall growth of a group of cells occurs. In most cases, then, growth is the result of an increase in cell number.