126 Applied Biophysics 



provides a very useful approximate estimate of the size of the 

 biological unit in cases where this unit may be inactivated by a 

 single ion cluster. It is interesting to note that, on the basis 

 of such studies. Lea and Salaman -^ put forward the view, 

 before an internal structure was demonstrated by electron micro- 

 graphs, that vaccinia virus should be regarded as a single-celled 

 organism containing a considerable number of discrete struc- 

 tural units analogous to genes. 



The Structural Changes Induced in Chromosomes by 

 Different Types of Ionizing Radiation 



The nature of the chromosome structural changes induced 

 by radiation is discussed in detail in another article. Many of 

 these structural changes are known to be injurious and some 

 to be lethal to the daughter cells, and they are produced by 

 relatively low doses of radiation — in the materials studied, the 

 doses employed have rarely exceeded 500 rontgens of X-radia- 

 tion, or a tenth of this dose of alpha radiation. There can be 

 little doubt, therefore, that they play an important part in the 

 response of many types of cell to radiation, including probably 

 the response of normal and malignant tissue to X-radiation 

 in certain types of radiotherapeutic techniques. ^*^ 



Before considering the influence of the type of radiation on 

 the response of cells, organisms, and tissues, it will be con- 

 venient to summarize the information regarding the chromo- 

 some structural changes. The production of a chromosome 

 break requires that a particle shall pass through (or in the 

 immediate vicinity of) the chromosome, leaving an adequate 

 number of ions within the chromosome. The exact number of 

 ions required probably varies from one type of cell to another, 

 and may well vary with the stage of development of any one 

 cell. Experimentally, it is found that high ion-density radiations 

 are more effective than low ones in breaking the pollen grain 

 chromosomes of the plant Tradescantia at prophase (figure 3), 

 and in fact, it appears that only radiation which produces at 

 least 200 ions per micron of track has a high break-producing 



