III. THE NATURE OF GENES 241 



visible (20). If isolated from the cytoplasm, the genes would be 

 unable to exert their influences upon the latter even when their active 

 groups were not folded. Anyhow, there should be in a cell a mecha- 

 nism in action by which continuous effect of genes is prevented. 



The mutation of genes may result from their irreversible struc- 

 tural change, and since chromosomes can be regarded as a kind of 

 aggregates or even crystals of genes, the shape of the chromosomes 

 composed of genes undergoing mutation may sometimes be altered. As 

 is generally recognized, the shape of chromosomes is occasionally 

 changed following a mutation of the organism. 



3. The Size of Genes 



The majority of workers consider that the dimensions of genes 

 are not very different from those of viruses or of fibrous molecules. 

 For instance, from genetic data it has been inferred that the shape of 

 the genes is rod-like and their length is approximately 125 (x, their 

 width being 5-20// (21). An electron microscopic study of salivary 

 chromosomes has shown that giant chromosomes were composed of 

 series of granules of 210-330 m/-< in diameter (22). 



On the other hand, a number of workers claim that genes must 

 be much smaller than viruses, because small as they are virus parti- 

 cles seem to be endowed with many component characters each of 

 which is inheritable independently, a fact already discussed in the 

 beginning of this chapter. 



If genes are regarded as entities as shown in Fig. 23, they must 

 have a dimension of viruses, but if only unit structures directing 

 each component character are taken into consideration, the dimension 

 will naturally appear much smaller. However, since unit structures 

 themselves cannot exist independently of the whole protein molecules, 

 it should be proper to regard the genes as virus-like polymerization 

 product composed of protein and nucleic acid. 



In Fig. 23 only three unit structures are shown, but in reality 

 the structures must, of course, be present in a much larger number. 

 Since a single molecule of globulin may be composed of more than 

 1,000 amino acids, if a single amino acid residue can behave as a strong 

 unit structure directing a character of an organism, more than 1,000 

 determinant structures may be able to exist and accordingly more 

 than 1,000 kinds of genes may appear to be present in a cell, because 

 in a single gene particle only one structure of them may act as a 

 directive site. 



Viruses can be regarded as genes, though not so differentiated. 



