300 Molecular Action of Ionizing Radiations / 1 6 .: 2. 



electrons. Although these levels are introduced briefly in Chapter 10 

 and are expanded upon in this chapter, a detailed treatment of atomic 

 interactions with radiations is beyond the scope of this text. 



Biologically important effects of ionizing radiations in living matter 

 often involve two molecular species, proteins and nucleic acids. This is 

 particularly true of the chromosomal changes, for chromosomes are com- 

 posed of proteins plus nucleic acids. Both types of compounds are 

 complex high polymers, occurring naturally. The effects of ionizing 

 radiation on simpler high polymers are easier to interpret ; these are dis- 

 cussed first. This information is then applied to the changes induced 

 in proteins and nucleic acids by ionizing radiation. 



Several different types of radiation produce ionization in all high 

 polymers, both natural and synthetic. These same types of radiation 

 produce ionizing effects in biological cells. Some of the physical 

 characteristics of such radiations, which include a, /3, and y rays, proton, 

 neutron, and deuteron beams, and X-ray photons, have been reviewed 

 in Chapter 10. The action of all of these radiations is associated with 

 ionization and the breaking of bonds. In certain cases, such as dried 

 protein films, the local ionization in the polymer is the only active 

 process. However, when proteins and nucleic acids are in solution, the 

 ionization of the solvent plays an important role. 



Ionizing radiations have clinical and pathological results. In 

 addition, they are a tool to study the physical and chemical properties of 

 proteins and nucleic acids, which is the basis for including this chapter 

 in this text. Sections 5 and 6 are concerned with the properties of pro- 

 teins and nucleic acids which can be discovered by bombardment of 

 dried films with ionizing radiations. 



2. Polymers, Proteins, and DNA 



Chemically, both proteins and nucleic acids belong to a general class of 

 compounds called polymers (or high polymers). As discussed in the 

 previous chapter, a polymer is made up of a repeating type unit (the 

 monomer), which is duplicated again and again; the repeating units 

 making up proteins are called amino acids. Similarly, there are repeating 

 units making up nucleic acids; their monomers are called nucleotides. 

 The chemical structures of both proteins and nucleic acids are presented 

 in Chapter 15. 



The amino acids are joined together by peptide bonds to form proteins. 

 Because a molecule of water is eliminated for each peptide bond formed, 

 it is customary to state that the amino acids condense to form the pro- 

 teins. Nucleic acids, too, are condensation-type polymers, a molecule 



