Induction of DNA double-strand breaks in mammalian cells and yeast.

Induction of DNA double-strand breaks (dsb) and their distribution are dependent on the energy deposition pattern within the cell nucleus (physical structure) and the ultrastructure of the chromosomes and its variation by the cell cycle and gene activities (biological structure). For electron radiation very similar RBE-values are observed for mammalian and yeast cells (AlK, 1.5 keV, 15 keV/micrometer: 2.6 in mammalian cells and 2.2 in yeast; CK 0.278 keV, 23 keV/micrometer: approx. 2.5 in mammalian cells and 3.8 in yeast). In contrast, the RBE-values for the induction of dsb of 4He2+ and light ions in the LET range from about 100 keV/micrometer up to 1000 keV/micrometer are significantly higher for yeast cells compared to mammalian cells. For example, the RBE-value of alpha-particles (120 keV/micrometer) is about 1.2 for mammalian cells whereas for yeast the RBE-value is about 2.5. The yeast chromatin has less condensed fibres compared with mammalian cells. Since a single CK photoelectron can induce only one dsb, the different condensation of the mammalian and yeast chromatin has no influence. However, particles may induce more than one dsb when traversing a chromatin fibre. The probability for the induction of closely neighboured dsb is higher the more condensed the chromatin fibres are. Since small DNA fragments (50 bp up to several kbp) are lost by standard methods of lysis, the underestimation of dsb yields increases with fibre condensation, which is in accordance with the observes dsb yields in mammalian cells and yeast. In order to obtain relevant yields of dsb (and corresponding RBE-values) the measurement of all DNA fragments down to about 50 bp are needed.

A natural history of athleticism and cardiovascular health.

Epidemiologic studies in Olympic year 1984 suggest that personal athleticism alters trends in life-style and coronary heart disease. Analysis of 572 first attacks among 16,936 Harvard alumni, 1962 to 1972, and 1,413 total deaths, 1962 to 1978, shows that habitual postcollege exercise, not student sports play, predicts low coronary heart disease risk. Sedentary alumni, even ex-varsity athletes, have high risk. Sedentary students becoming physically active alumni acquire low risk. Exercise benefit is independent of contrary life-style elements--smoking, obesity, weight gain, hypertension, and adverse parental disease history--in affecting coronary heart disease incidence. Hypertension is clinically the strongest predictor of coronary attack, but inadequate exercise is strongest on a community basis. Exercise level is inversely related to total, cardiovascular, and respiratory mortality but less related to cancer or unnatural deaths. The current exercise revolution may improve life-style, cardiovascular health, and longevity.