400 AN INTRODUCTION TO MODERN GENETICS 



mum thickness of the chromosome thread (p. 378). It is therefore 

 impossible to reject, from consideration of gene reduplication, the idea 

 that the gene is a single unit. On the other hand, a further difficulty 

 arises in this connection, namely the necessity to find some mechanism 

 which accoimts for the fact that only two genes, the old one and the 

 new, are present at the end of each intermitotic period. The reduplica- 

 tion occurs only once. No plausible hypothesis to account for this has 

 been put forward. 



Alternatively, we may assume that the gene is compound, consisting 

 of a number of identical subunits. Such a supposition probably simpHfies 

 the task of accounting for gene reproduction. The chemical forces on 

 which the identity of the new and old gene depend would not have to 

 extend so far from the radicals to which they were due, since the 

 thickness of the subunits would be less than that estimated for the whole 

 chromosome thread. Similarly the reproduction might continue gradu- 

 ally, and the gene grow until it eventually spHt into two by reason of 

 some instability which increased with increasing size, such as that 

 which causes a drop to break up when it passes a certain size limit. 

 The difficulty of this hypothesis, as was pointed out before, is the fact 

 that some genes (though only a fev*') show more or less equal rates of 

 back and forward mutation. 



It appears not imlikely that nucleic acid plays some important role 

 in the process of gene redupHcation. For instance, the most rapid 

 synthesis of nucleic acid occurs just before the prophase of mitosis, 

 at the time when the chromosome appears to spUt or reduplicate. 

 Again, it is remarkable that the virus proteins, which share with the 

 genes the property of identical reproduction in living systems,^ and of 

 mutation also,^ contain large quantities of nucleic acid. Conceivably 

 there is some connection here with the remarkable fact recendy revealed 

 by Schultz and Caspersson,^ ±at nucleic acid is in some way connected 

 with the stabiUty of the gene; when parts of the inert region in Droso- 

 phila are translocated into the euchromatic regions, they frequently 

 cause the neighbouring loci to become unstable and undergo somatc 

 mutations which give rise to phenotypic spotting such as that found 

 with other mutable genes; and this instabihty appears to be correlated 

 with an increase in the nucleic acid content of the corresponding bands 

 in the salivary gland chromosomes. 

 All the above considerations apply to genes considered as units of 



' Cf. Stanley 1938. 



^ Cf. McKinnery 1937. ^ Schultz and Caspersson 1938. 



