The Differential Capacity for Rna Synthesis among Chromosomes: a Cytological Approach.

It is an established concept that in any given cell or tissue of a multicellular organism, only a part of the total genetic information is used. Although the exact mechanism has not been elucidated, it is presumed that reversible and irreversible inactivations of genetic loci with respect to RNA synthesis are the controlling factors. Hoyer and McCarthy' have demonstrated with DNA/RNA hybridization experiments, that different mouse tissues produce different messenger RNA's. Lyon2 proposed that one of the X chromosomes of the female mouse is inactive in somatic tissues; support for this hypothesis has come from human genetic studies.' The genetic inactivity of one X chromosome in female mammalian cells agrees well with the cytological observation that one X chromosome is heteropyknotic.4 Some evidence is available to show that heteropyknotic chromatin is low in RNA synthetic activity.5, 6 Autoradiographic studies on DNA replication patterns, using tritiated thymidine (H3-TdR), have shown that the heteropyknotic X chromosome in female mammals is late-replicating.7 8 It is now thought that heteropyknosis and late replication are two manifestations of an inactive genetic region.9' 10 A number of investigators have claimed that H3-TdR causes chromosomal aberrations.11-'3 The average tritium beta particle will produce 10-20 ionizations in the first 0.1 1L of its path. According to the calculations of Lea,14 15-20 ionizations are required within this distance to produce a chromatid break with a probability of 1. Since TdR is a precursor of DNA, damage to the chromosomes is conceivably related to incorporation of the isotope at a particular region. With background knowledge of the DNA replication sequences among the chromosomes of the Chinese hamster,'5 Hsu and Zenzes'6 have reported that chromosome breakage with H3-TdR is nonrandom, and correlates well with the site of isotope incorporation as well as with the stage of DNA synthesis at which the isotope is introduced. Since H3-TdR is permanently incorporated into the DNA of test systems, chromosomal damage caused by disintegration of the isotope should be considerably greater than that induced by tritiated uridine (H3-UR), an RNA precursor, which is in proximity to the DNA template only momentarily. However, in active genetic loci, a particular message is probably synthesized more than once during a cell cycle, so that when a high dose or prolonged treatment is employed, some chromosomal damage would be expected. Bender et al."7 have demonstrated that HI-UR is capable of producing chromosome damage in human leucocytes. It may be possible to use the damage produced by H3-UR in different chromosomes or chromosome regions as a monitoring system for RNA synthetic activity. If a chromosome is allegedly inactivated in a certain cell species, it should incorporate little H3-UR and thereby sustain little damage. However, damage can be inflicted on the same chromosome by H3-TdR if the labeling time coincides with its DNA synthetic period. A prediction can be made that differential damage