RADIATION DAMAGE TO CHROMOSOMES 205 



production without effect on the kilHng rate of the radiation. This is a 

 rather significant observation l)ecause it sho\\'s that true protection by 

 chemicals exists against mutation ])ro(kiction without dependence on 

 the survival ratio. 



Another project in om- laboratory is the study of liow chromosomal 

 breaks can be modified by treatment after exposure to radiation. 

 One of our associates, Sheldon Wolff (1900). has found that he is able to 

 modify the rejoining of broken chromosomes either by inhibiting pro- 

 tein synthesis with chloramphenicol or by enhancing the rejoining of 

 chromosome breaks with an energy source like adenosine triphosphate 

 (ATP). This is possible in chromosomes since the breaks stay open after 

 exposure to X-rays for about 30 or 40 min. Different types of treatment 

 are possible during this jieriod. This is a very significant field since it 

 shows us we can manipulate irradiation damage by different methods. 

 I am sure these studies will lead to many other interesting observations 

 on effects of radiation on living cells. 



The work of R. F. Kimball et al. (1959, 1960) of our labora- 

 tory has also been concerned with radiation damage. We observed in 

 our laboratory years ago that radiation damage can be reduced very 

 significantly by treatment after radiation. At that time we used methods 

 which inhibited cell division but which still permitted metabolism or 

 enzyme activity to take place. The work was done mostly in regard to 

 survival, but later on was extended to mutation production. This work 

 has now been verified in a very striking way by Kimball in regard to 

 the repair of jore-mutation damage in Paramecium. His studies again 

 demonstrate the ability of cells to repair a good part of damage. He 

 observed that the number of recessive lethals and slow-growth muta- 

 tions produced in Paramecium were reduced by a factor of two by post- 

 irradiation treatment by either chloramphenicol, streptomycin, or 

 caffeine and also by starvation. But again, as has been pointed out 

 previously, the amount of mutation can be modified only if the post- 

 treatment is begun before chromosome duplication takes place. It 

 happens that agents which produce this post -treatment effect also 

 delay chromosome duplication. Thus more time is made available be- 

 tween irradiation and duplication for repair of the damage; conse- 

 quently less mutation is produced. The rate of this change in mutation 

 is decreased by starvation and by various metabolic inhibitors. This 

 verifies againthe early observation that radiation damage is not definitely 

 frozen but that by careful analysis and careful investigation of different 

 factors involved, mutation production and survival can be manipulated 

 in a very definite way. Careful analysis by Kimball pinpoints the period 

 in which this repair can take place, i.e. especially before chromosome 



