A linear dose response was observed for radon-induced mutations at the CHO-hprt locus with an induction frequency of 1.4 x 10(-4) mutants per viable cell per gray. Mutants isolated after two levels of radon exposure were evaluated using Southern blot techniques and polymerase chain reaction (PCR) exon amplification. No significant differences in mutational spectra were detected at these two exposure levels. Of 52 radon-induced mutations, 48% sustained a gene deletion, 23% underwent a rearrangement of the banding patterns or loss of one or more exons, and 29% showed no change from the parental line. These mutants were compared with mutants produced after X irradiation (3 Gy) and with spontaneous mutants from untreated cells. The spectra of mutation types in cells treated with radon and X rays were not significantly different. In contrast, 31 spontaneous mutations exhibited a low percentage of gene deletion events (16%); most spontaneous mutants showed no change (74%); the remaining 10% were classified as alterations. In conclusion, the principal lesion seen at the CHO-hprt locus after radiation exposure is gene deletion, while the predominant class of spontaneous mutations is composed of smaller events not detectable by Southern blot or PCR exon analysis.

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We have used viscoelastometry in -an attempt to understand the physical organization of genetic material in Tetrahymena nuclei. The micronucleus or germ line nucleus is diploid. It divides mitotically during vegetative growth, and five pairs of chromosomes are seen in meiosis. The macronucleus, or somatic nucleus, is approximately 45-ploid, divides amitotically, and has no visible chromosomes at any stage. Viscoelastic analysis of Tetrahymena macronuclei reveals DNA molecules of 2-3 X 1010 daltons accounting for much, if not all, of the macronuclear DNA. Since the average chromosome in the micronucleus contains 2.4-2.7 X 1010 daltons of DNA, we deduce that the macronucleus of Tetrahymena contains chromosome-sized DNA molecules. The ciliated protozoa, Tetrahymena, are remarkable for their nuclear dimorphism; they possess a transcriptionally inactive, diploid, germinal micronucleus, and a transcriptionally active, polyploid, somatic macronucleus. During vegetative growth the micronucleus divides mitotically while the macronucleus divides amitotically. Sexual conjugation results in degeneration of the macronucleus while the micronucleus undergoes meiosis. Subsequent reciprocal exchange and fusion of two haploid gametic nuclei maintains the genetic continuity of the organism. From a daughter of the zygotic nucleus formed during conjugation a new macronucleus arises by amplification of the genetic material. Presumably, before macronuclear differentiation occurs the micronucleus and the macronuclear anlage contain identical genetic material. However, both go on to develop into quite different organelles with distinctly different functions. The mechanism by which Tetrahymena establish and maintain nuclear dimorphism remains an intriguing unsolved problem.At the present time the genetics and molecular architecture of the nuclei of Tetrahymena are being intensively studied (for reviews see refs. 1-4). The diploid micronucleus contains five pairs of approximately equal-sized chromosomes clearly visible during conjugation. Genetic markers on this germinal nucleus are distributed in the normal mendelian fashion. The macronucleus, by comparison, does not contain visible chromosomes, but rather many "chromatin bodies," and genetic markers are apparently segregated randomly during vegetative fission. On the basis of cytological (5), genetic (6), and cytospectrophotometric (7,8) evidence, the macronucleus appears to contain 45 haploid genomes after cell division, with each genome containing at least 90% of the sequences found in the micronuclear genome (9).Of central importance to an adequate understanding of Tetrahymena nuclear dimorphism is an explanation of how DNA is organized within the two types of nuclei. Conceivably, micronuclear DNA resembles DNA from other organisms in which chromosomes are visible, such as Drosophila (10) and yeast (11,12), in which chromosome-sized DNA molecules are observed, or dinoflagellates (13) and mice (14), in which large molecules somewhat smaller than chromosome size are observa...

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