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Because of the redundancy of ribosomal RNA cistrons and in analogy with a few known special cases, one may postulate that a few "master genes" for rRNA direct the synthesis of multiple copies of themselves per cell cycle. We have shown that in Chinese hamster cells rRNA cistrons duplicate like the bulk of DNA, once in a cell cycle. The time during the S-phase a t which rRNA genes duplicate has also been determined by means of RNA-DNA hybridization studies.The time of duplication of rRNA genes occurs between t = 1.5 and t = 3.0 hours from the beginning of the 8-phase.Some cells requiring a very high rate of rRNA synthesis are known to cope with this situation by an extrachromosomal amplification of the rRNA genes. This appears to be the case for the oocytes of Xenopus laevis as described by Brown [I], in which the extra genes appear to be localized in a large number of newly formed nucleoli. This phenomenon is not only restricted to amphibia but seems to occur also in oocytes of the insect Ditiscus [Z, 31 in the latter case these rRNA genes amplification leads to the formation of a giant nucleolus usually known as the Giardina body. This extra duplication of ribosomal RNA genes is probably directed by chromosomal "master genes". rRNA cistrons, however, are normally redundant in somatic cells of these organisms and of all other organisms so far studied, in fact, even the bacterial cells show a fair amount of redundancy for the cistrons that code for rRNA. More precisely, from the percentage of DNA that can form hybrids with homologous rRNA it has been found that in E. coli, in Drosophila and in mammalian cells the redundancy reaches a value of 10, 260 and a few hundreds respectively [4-71.It seemed possible that the extrachromosomal amplification found in oocytes of X . laevis and Ditiscus is nothing but a magnification of a more general phenomenon to be found in all somatic cells; the redundant rRNA cistrons, even if localized on chromosomes might be formed in the cells by a series of duplication of one or few "master genes" although not in such proportions as to give rise to visible modification of the cells.If this were the case, it would provide an easy explanation for the observation that ribosomal RNA, although coded by tens and even hundreds of cistrons, seems very homogeneus in single cells or organism within the limits of the analytical techniques so far employed [S-101. By the use of density labeling of DNA and RNA-DNA hybridization techniques we have studied the mode of duplication of rRNA cistrons in exponentially growing Chinese hamster cells. The results are consistent with the view that these cistrons duplicate once in a cell cycle thus eliminating the "master gene" hypothesis as defined above. The explanation for the homogeneity of rRNA must be sought elsewhere.It occurred to us that RNA-DNA hybridization could be a very useful tool to determine the time of the S-phase a t which cistrons for rRNA duplicate. We were confident that such time could be determined as DNA synthesis has been reported to b...

Deoxyribonucleic acid contains sequences complementary to homologous amino-acid transfer ribonucleic acid molecules which serve as the translating device between polyribonucleotides and proteins. This implies that the RNA molecules have their primary origin in DNA. From the amount of DNA participating, one would infer that more than 20 complementary sequences exist per genome, a conclusion consistent with a degenerate code. The fact that complex formation occurs most readily with homologous RNA suggests that, while the language remains universal, each dictionary is uniquely identifiable with its own genome.

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