POLLARD AND KENNEDY 137 



ber of loci. The results are only preliminary, but they do contribute to an 

 emerging picture of cellular processes which is worth some consideration. 



Closely related to this work is a series of studies by Billen and Volkin [1], 

 Billen and Lichstein [2], Billen, Stapleton, and Hollaender [3], and Billen [4] 

 on the effect of X rays on several factors in E. coli. Their results will be 

 discussed later. Some work by Labaw, Mosley, and Wyckoff [9] is also of in- 

 terest, though directed at bacteriophage development. 



PRINCIPLES OF RADIATION INACTIVATION STUDIES 



For these experiments the simplest and most available source of ionizing 

 radiation, X rays, was used. X rays produce ionizations which are distributed 

 along the tracks of secondary electrons, light particles that ionize relatively 

 sparsely. In our arrangements, where 250-kv X rays filtered through 1 mm of 

 aluminum were used, the average secondary-electron energy is in the neigh- 

 borhood of 50 kv, and so produces ionizations which are generally separated 

 by distances of over 1000 A. Since the electrons scatter readily, the net effect 

 is to produce local energy releases, which average 110 ev each, at random 

 throughout the bacterium. The number per unit volume depends on the dose 

 of radiation, and for the purposes of this paper will be taken as 5 X lO^/cc/r, 

 which assumes a density of a little over unity for the bacterium. 



Many previous studies, quoted in the preceding paper of this volume, have 

 shown that the energy releases can remove biological potency from quite large 

 biological molecules. In proteins it is thought that the reason is the migration 

 of the positive charge left by ionization, along a covalently bonded structure 

 until it reaches a weak point, where chemical action by water or oxygen can 

 cause a destructive chemical change. Then ionization anywhere in the cova- 

 lently bonded structure is destructive. For nucleic acids the best evidence sug- 

 gests that either a break in the chain can result, or a cross linking of two chains 

 can take place, both drastic events. In any event, the philosophy of these ex- 

 periments is to estimate the chance of complete escape from any ionization at 

 all, arguing that the chance of being ionized and escaping any effect is statisti- 

 cally too low to be worth considering. Such estimates are, of course, tentative 

 to some degree. Nevertheless, the sum of 10 years' work in our laboratory, to- 

 gether with the findings of many others, renders them plausible. 



The structure that is being bombarded by these random energy releases is 

 shown in the section electron micrograph in figure 1. The section is of two 

 Escherichia coli bacteria, one of which has passed into a rather filamentous 

 form, and the other of which is normal. The sectioning and microscopy are 

 entirely due to L. Caro of our laboratory, and the illustration is presented by 

 his permission. It shows clearly the presence of an inner region, almost cer- 

 tainly the locale of the DNA, as has been shown by Caro, Van Tubergen, and 

 Forro [5], and an outer region which contains microsomal particles, or "ribo- 

 somes," and which is presumed to be the region of protein synthesis. Figure 2 



