Genes — Nature and Consequence 



445 



we cannot use the origin of the DNA primer 

 as part of a proof that it is genie. But, con- 

 sider certain chemical characteristics pos- 

 sessed by the DNA primer. It is self-replicat- 

 ing under in vitro conditions, and, moreover, 

 replicates precisely certain chemical modi- 

 fications of itself. This is demonstrated 

 directly by the extensive synthesis which oc- 

 curs after uracil is substituted for the thymine 

 in the substrate (cf. p. 325), and is demon- 

 strated indirectly, using the normal substrate, 

 by the synthesized product having the same 

 A + T/C + G ratio as the primer, regard- 

 less of what variation in this ratio the primer 

 may show. We conclude, therefore, that 

 DNA synthesis in vitro fulfills one of the re- 

 quirements of our definition of genetic ma- 

 terial. The detection of genetic material was 

 originally dependent upon its presence in 

 organisms and its production of a pheno- 

 typic effect. In the course of this book, how- 

 ever, we have apparently dispensed with these 

 requirements. Pure virus DNA in a test 

 tube is considered genetic material, even 

 though it is no longer intraorganismal, re- 

 combining, mutating, replicating, or per- 

 forming any phenotypic function. This is 

 valid on the basis that this DNA either is 

 known or expected to possess such proper- 

 ties when introduced into an organism, or is 

 chemically indistinguishable from organis- 

 mal DNA known or expected to have these 

 properties. The DNA synthesized in vitro 

 is chemically indistinguishable from organis- 

 mal DNA, is capable of self-replicating itself 

 and some of its modifications, and of under- 

 going strand separation and recombination. 

 Since it possesses these characteristics it 

 would seem reasonable to consider that it, 

 too, is genetic material, even though, until 

 now, attempts to detect a cistronic effect via 

 transformation have been unsuccessful. Ac- 

 cording to this view, then, genetic material 

 has already been synthesized in the test tube. 

 We have just reasoned that DNA may be 

 identified as genetic material, using solely 



the operation of chemical investigation. 

 Using living cells there are other operations 

 which can be employed to study and identify 

 genes, including recombination, mutation, 

 and phenotypic function. These other 

 methods of studying genes have led to our 

 present understanding of the recon and cis- 

 tron. Chemically, we have found that the 

 recon may be a single nucleotide, while the 

 cistron is composed of several or many 

 recons. What is the smallest chemical unit 

 of genetic material capable of (chemical 

 and informational) replication? We do not 

 know. 



The phenotypical, mutational, and recom- 

 binational operations employed to study 

 gene properties are different from the study 

 of genes by chemical methods in two respects. 

 First, they require that the gene produce a 

 phenotypic effect. Second, they require a 

 genetic alternative which produces a detect- 

 able change in phenotype. Let us make 

 some additional observations regarding each 

 of these three operations in turn. 



It should be made clear that just because 

 all three of these operations depend upon 

 phenotypic effects for their detection does not 

 necessarily mean that these operations auto- 

 matically characterize cistrons. For cistrons 

 are genetic units as defined from the study 

 of what are apparently the single primary 

 phenotypic effects of genes. If we study the 

 mutation from dull red to white eye in Dro- 

 sophila, for example, or the recombination 

 of such mutants, we are not necessarily con- 

 cerned with whether we are dealing with the 

 primary effects of genetic units; merely 

 studying the effects of genes undergoing 

 mutation and recombination does not auto- 

 matically reveal information on how cistrons 

 are related to the traits produced. The cis- 

 tron has meaning only when it is hypothesized 

 that the genetic material has a single effect 

 other than replication, and only after the 

 decision is made as to which effect is to be 

 considered primary can the scope of the cis- 



