280 Harold J. Morowitz 



by Mr. J. Edward Donnellan of the Yale Biophysics Department, and indicates 

 inactivation cross-section for colony formation as a function of LET. From 

 the electron irradiations of Proctor et al. (7), the target volume for these spores 

 is of the order of 10"^'^ cm^. However, this seems to be the volume of the sub- 

 structure with the highest information density. For we see that as we increase 

 the LET the radiation rapidly becomes more efficient in causing inactivation. 

 What we are doing in increasing LET is to increase the probabihty of several 

 ionizations in a given substructure of the spore. Since the cross-section then 

 rises so dramatically, we must conclude that targets of lower information density 

 than the one originally inactivated at low ion densities are now coming into 

 play. Since the curves are exponential, the multiple ionizations in any sub- 

 structure must be coming from the same fast charged particle. Under these 

 circumstances, x must of necessity be small compared with M, and the secondary 

 targets must still retain an information content near //max, if we ignore energy 

 transfer. 



We may conclude, in general, that any large structure which is capable 

 of being inactivated by the passage of a single fast charged particle through 

 that structure probably has an information content which is an appreciable 

 fraction of /^max- In general, if information can be transferred with high 

 efficiency over g atoms, H is probably greater than H^^aJ^lgf. 



There seems to be a possibility of reducing energy transfer and thus getting 

 a better estimate of information content. When enzymes are irradiated with 

 fast charged particles and the experiments are carried out at different tempera- 

 tures, the target cross-section is found to be an increasing function of temperature 

 (3). The possibility exists that energy transfer is being reduced at the low 

 temperatures, and data taken in this range might provide a better index of 

 the actual information content. However, considerations of this type demand 

 a thoroughgoing analysis of the physics of the situation. 



Another method of random disordering exists which might provide an 

 even more powerful tool for the elucidation of information content. It has 

 been recently shown (8) that viruses labeled with P^^ lose activity on standing, 

 and the rate of loss is associated with the amount of P^^ incorporated. Now 

 the decay of a radioactive atom incorporated in a biological structure, and the 

 consequent transmutation of the atom, represents a random disordering. 

 If the labeling is random, the rate of decay should provide a measure of the 

 fraction of atoms of the labeled type which require precise specification in 

 order for the structure to be functional. Such an information evaluation should 

 be possible for phosphorus, sulfur, hydrogen, carbon, sodium and calcium. 

 Thus we may inquire about a complicated structure like a spore: how many 

 of the phosphorus atoms in the structure are required to specify a functional 

 unit? Experiments and calculations of this type should serve to limit the value 

 of the information content of biological structures. 



REFERENCES 



1. H. J. Morowitz: Some order-disorder considerations in living systems. Bull. Math. 

 Biophys. 17, 81-86 (1955). 



2. E. C. Pollard and A. E. Dimond: The inactivation of tobacco mosaic virus by ionizing 

 radiation. Phytopathology A6, 214-218 (1956). 



