THE SYNTHESIS OF NEW SUBSTANCES 385 



the particles moved under these forces would seem to be comparable 

 with those usually considered as microsomes. The role played by micro- 

 somes in the synthetic activity of echinoderm embryos, and their relations 

 with the mitochondria, have been discussed earher (p. 90). 



It will be seen that neither in Caspersson's nor in Brachet's theories is 

 it supposed that the synthesis of proteins occurs actually at the gene inside 

 the nucleus. If enzymes are sometimes produced in this way (p. 355), 

 which is by no means certain, that can hardly be the general rule of pro- 

 tein synthesis. One suggestion has been made (e.g. Wright 1945, Wad- 

 dington 1939^), however, which minimises the difference between syn- 

 thesis at the gene and synthesis in the cytoplasm. According to this, the 

 genes directly produce substances which are replicas of themselves in 

 most respects (or possibly in all except for the coimecting links which 

 hold the genes together in the chromosome); and it is supposed that 

 these repHcas pass into the cytoplasm and there control the synthetic 

 processes. This hypothesis has the merit of simpHcity, but the evidence 

 for it is slight; the fact that synthetic activity disappears or diminishes 

 when the nucleus is removed suggests that the postulated gene-rephcas 

 cannot at all fully take the place of true nuclear genes and it is therefore 

 likely that they cannot actually be rephcas in the full sense of the word. 

 It may very well be the case, however, that the immediate products of 

 gene activity have a considerable chemical resemblance to the genes or 

 to some part of them. Possibly the genes produce, as a first step, a product 

 which resembles the protein which, in the chromosome, is combined 

 with DNA, and this, passing into the cytoplasm, becomes associated with 

 RNA, either near the nuclear membrane as Caspersson suggests or in the 

 microsomes of Brachet. 



It will be seen from the discussion in this chapter that in spite of the 

 very large effort which has been devoted recently to the attempt to dis- 

 cover how genes operate, and the important body of interesting results 

 which have been gathered, there has really been rather little progress 

 towards the solution of the problem which has been put in the centre of 

 the stage. This may be due to the inherent difficulties of the field; but the 

 biologist who is not primarily a biochemist may be tempted to wonder 

 whether the problem is not perhaps being envisaged in too simple terms. 

 We have tended to think both of genes and of their products as definite 

 and discrete particles, of, perhaps, the dimensions of a protein molecule 

 or a little more. Possibly the active participants in gene-operations are 

 actually of a higher order of complexity than this. As Goldschmidt has 

 urged, and as we have seen above, there is considerable evidence which 

 could be taken to support the idea that the active units in chromosomes 



