DNA synthesis is limited to interphase and frequently to a part of the interphase stage. When only a part of interphase is involved, DNA synthesis may occur at the beginning of interphase, near the middle of interphase or at the end, depending on the tissue and the species. DNA synthesis and chromosome replication occur simultaneously and probably are part of the same sequence of events. Chromosomes become double to radiation breakage as the DNA is synthesized and the basic proteins of the nucleus increase at the same time. RNA synthesis is hardly detectable in the cell during periods of DNA synthesis and also stops or decreases markedly during late stages of division when the chromosomes are condensed. Although this may indicate direct control of the chromosomes in the synthesis of RNA, other interpretations are possible. By utilizing tritium-labeled thymidine which is a selective label for DNA, and therefore the chromosomes, the distribution of original and new DNA can be determined in individual chromo...