352 RADIATION BIOLOGY 



these facts are to be found in the properties of the genetic material itself. 

 These therefore call for a brief preliminary review. 



1. FUNDAMENTAL PROPERTIES OF THE GENETIC MATERIAL 



Genetic studies on widely diverse organisms have shown that the bio- 

 chemical operations of the cell, ranging from the most fundamental to 

 the most trivial, and from the "physiological" to the "morphogenetic," 

 depend in the last analysis upon the nature of the self-reproducing nucleo- 

 proteins that have been provided. These are, for the most part at least, 

 contained in the chromosomes, although in plants of varied kinds and in 

 some microorganisms of an animal nature some genes have been proved 

 to be present in certain cytoplasmic particles. A chromosome has been 

 proved to consist essentially of a fine desoxyribonucleoprotein thread, 

 thousands of times longer than thick, differentiated along its length into 

 hundreds or thousands of functionally distinct and individually self- 

 reproducing regions (probably constituting discrete segments), the genes. 



Each chromosome, and in fact each gene in it, considered separately, 

 is properly termed self-reproducing, inasmuch as it possesses the prop- 

 erty, when in its natural protoplasmic medium, of so guiding the selection 

 and assemblage, possibly the reconstruction and shaping, and certainly 

 the bonding together, of surrounding raw materials, as to result in the 

 construction, next to itself, of an exact copy of itself. Moreover, and most 

 important of all, even if it has undergone some permanent change (muta- 

 tion) in its own inner configuration, it will now guide the next synthesis 

 so that this very change itself also becomes incorporated in the new copy. 

 This property has been called "convariant reproduction." 



As a result of this ability to reproduce its own changes, each gene must 

 in the course of ages have undergone an extensive evolution, involving a 

 long series of mutational steps that gave it an increasingly complex organ- 

 ization, more nicely adapted biochemically to serving given needs of the 

 organism. Since mutations are not designed in advance for useful ends, 

 the great majority are necessarily detrimental to survival, and the organ- 

 isms inheriting these tend to die out. However, the organisms with the 

 relatively rare mutations that happen to be helpful tend to multiply and, 

 because of the abihty of the gene to pass on the mutant pattern to its 

 daughter genes, these organisms produce descendants inheriting the 

 advantageous mutation. In addition to this slow step-by-step evolution 

 of each gene there has been, according to modern genetic theory, an even 

 slower, stepwise increase in the number of kinds of genes. The initial 

 event of such a step usually consists in a part of a chromosome, containing 

 a group of genes, becoming detached from its place and inserted into a 

 new position into some chromosome or set of chromosomes which already 

 has these same genes in their old positions as well. This process of form- 



