190 CELLS, TISSUES, AND ORGANISMS 



formation of chromosomal bridges at doses in the range of two or more 

 D° values is perhaps the most direct experimental verification of this 

 picture. (3) The chromosomal hypothesis is able to explain satisfac- 

 torily the high sensitivity of mammalian cells to X-irradiation, as com- 

 pared to the lower sensitivity of microorganisms such as bacteria, 

 yeasts, and viruses (Puck, 1959). (4) All cell functions so far studied, 

 other than those directly involving the functions of the chromosomes 

 and their contained genes (such as mitosis and DNA synthesis), have 

 been found to be tens to hundreds of times more resistant to X-irradia- 

 tion than cell reproduction is. In this category are included the abilities 

 of mammalian cells to transport actively the vital dyes, to metabolize 

 glucose, and to biosynthesize specific viruses. (5) The chromosomal 

 picture originally was proposed on purely theoretical grounds which 

 included, among others, the multiple-hit nature of the S3 survival 

 curve. Scott later predicted on this basis that cells irradiated with 

 doses sufficient only to encompass the shoulder region of the survival 

 curve would, if allowed to recover before a subsequent irradiation, re- 

 quire virtually again as much radiation to be killed as if the preliminary 

 exposure had not taken place. Exactly this behavior was demonstrated 

 by Elkind in a quantitative study of the irradiation of Chinese hamster 

 cells in vitro. (6) Elegant experiments by Painter and his co-workers 

 have recently demonstrated that mammalian cell reproduction is ex- 

 ceedingly sensitive to destruction by the radiation resulting from 

 tritium-labeled thymidine, but only when the tritium is actually incor- 

 porated into the nuclear DNA ( Drew and Painter, 1959 ) . 



In addition to reproductive killing, which is essentially irreversible, 

 ionizing radiation is known to produce a delay in mitosis which is 

 temporary and presumably leaves no permanent change in the cell so 

 affected. It has been assumed that this reversible eflFect on cell division 

 involves an action of the radiation on non-genetic structures. However, 

 our recent studies make it at least equally probable that this re- 

 versible reproductive delay also results from a primary damage to 

 chromosomal structures. The nature of this evidence is as follows: (1) 

 Studies on mammalian cells indicated that exceedingly small doses, in 

 the order of 25 to 50 roentgens, are sufficient to cause demonstrable, 

 temporary, mitotic inhibition in the majority of a cell population 

 cultured in vitro. The small size of this dose indicates that the volumes 

 of the sensitive sites involved must be quite large, and comparable to 

 the chromosomal volume. (2) Disappearance of mitotic figures in cells 

 irradiated with such low doses occurs within 30 minutes or less after 

 administration of the radiation. This would appear to indicate that the 

 cells affected are in the pre-mitotic state. (3) The number of chromo- 

 some breaks observed in irradiated normal human cells is maximal in 

 the period immediately folowing radiation. Such cells display a break 



