THE SYNTHESIS OF NEW SUBSTANCES 373 



particles provide a model of repeat units of a still larger size. The smallest 

 recognisable units in chromosome structure (the bands in salivary gland 

 chromosomes) are still larger. One can represent such a sequence by a 

 series of letters such as a h c d e' f g' h' i' /" k' V m n o P Q R S T U 



V W X'. Here the individual letters represent the smallest repeat unit. 



The groups which are plain, dashed or underlined represent the next 

 largest and lower-case and capital letters represent a still longer periodi- 

 city. In a protein structure of this kind there are many types of units, and 

 different ways of defming the gene may lead to different results. We 

 might in some cases fmd that all the lower-case letters were acting as one 

 unit, while in other circumstances it might be the dashed lower-case 

 letters which behaved separately to the undashed ones. There might even 

 be an overlap between genes, in some cases the lower-case and the capital 

 letters behaving as two miits, while in other processes it was the under- 

 lined letters which went together. The usual crossover gene is thought 

 to contain some three hundred of the small polypeptide links so there is 

 plenty of room for complications of this kind. The nucleic acid is also a 

 linear structure, and it seems likely that in it too the order in which the 

 constituent groups are repeated can determine structures at least as com- 

 plex as those of the proteins (Davidson 1954, SEB Symposium 1947). It 

 may well be, indeed, that the factor which operates as a gene is a certain 

 arrangement of chemically reactive places, which may sometimes be 

 incorporated in a length of protein molecule, at other times in a nuclear 

 acid fibre and at other times in the combination of the two. For our 

 present purpose of investigatitig the nature of the reactions in which 

 genes participate, we may be content to keep in the back of our minds 

 the ambiguities in the precise meaning of the word and to understand 

 it as referring to some sort of small section of chromosome which is 

 acting as a unit. 



One line of investigation into the activities of genes has attempted 

 to discover something of their chemical nature by a study of artificial 

 mutation. It was shown by MuUer in 1927 that penetrating ionising 

 radiation increases the frequency with which genes mutate; and Auer- 

 bach and Robson in 1946 found that certain chemical substances have a 

 similar effect. In spite of the enormous amount of work which has fol- 

 lowed the lead of these pioneer investigations little has been discovered 

 which is really pertinent to our present problem. It has become clear that 

 genes can be brought into a condition of instabihty by various treatments, 

 but the effective physical and chemical agents are of kinds which can be 

 expected to affect a large range of different structures, so that the fact that 

 they are active in stimulating mutations does not make it possible to draw 



