Stanford Medical Bulletin 



replicated in step (3) concurrently with pro- 

 tein synthesis, in addition t<> its initiation from 

 DNA. 



The chief difference in primary structure be- 

 tween 1 )NA and RNA is the hydroxylation of 

 I .1 the ribose, so that a reactive sugar hy- 

 droxyl is available in RNA. This may prove 

 to be important in the less ordered secondary 

 structure of RNA. and in its function as an 

 intermediary to protein. It remains to be de- 

 termined whether the aminoacyl rrucleotidates 

 are esterified at <. V or at C ' which is also avail- 

 able in the terminal residue. From this resume 

 we may observe that the DNA backbone con- 

 stitutes m\ inert but ngul framework on which 

 the differential nucleins are strung. Their spa- 

 tial constraint lends specificity to the pattern 

 of hydrogen bonding exposed at each level. 

 This extended pattern is a plausible basis for 

 replication; it is difficult to visualize any re- 

 agents besides other nucleotides to which this 

 pattern would be relevant. These conditions 

 are quite apt tor a memory device — rubber and 

 guncotton are poor choices for a computing 

 tape. 



DNA AND BACTERIAL MUTATION 



The ignis fatuus of genetics has been the 

 specific mutagen, the reagent that would pene- 

 trate to a given gene, recognize and modify it 

 in a specific way. Directed mutation has long 

 been discredited for higher organisms and the 

 "molar indeterminacy" of mutation estab- 

 lished both for its spontaneous occurrence and 

 for its enhancement by X-rays (68). However, 

 the development of resistance apparently in- 

 duced by drugs revived illusions that bacterial 

 genes might be alterable, an inference that 

 would inevitably undermine the conception of 

 "gene" for these organisms. No wonder that 

 the mechanism of drug resistance has excited 

 so much controversy (89)! 



What sort of molecule could function as a 

 specific mutagen, a reagent for a particular one 

 ot the bacterium's complement of genes, 

 which can hardly number less than a thousand 

 targets 5 On the nucleic hypothesis, the small- 

 est segment capable of this variety would be a 

 /;ev</nucleotide, all possible configurations of 

 which must be discriminated by the specific 

 mutagen. How could this be generally accom- 



s- 



plished except by another molecule of con- 

 forming length and periodicity, that is, an 

 analogous polynucleotide? Certainly there is 



nothing in the chemistry of penicillin or strep- 

 tomycin to support their direct intervention in 

 nucleic instructions. 



In addition, we recognize no chemical re- 

 agent capable of substituting one nuclein for 

 another in the structure of existent DNA. 

 However, as the modification of a nuclein, 

 even to give an unnatural base, could have 

 mutagenic effect, the chief limitation for spe- 

 cific mutagenesis is the recognition of the ap- 

 propriate target. 



Of course the origin of drug resistance, for 

 all its theoretical implications, poses an experi- 

 mental challenge of its own. Concededly, ex- 

 periments cannot decide untried situations. 

 Nevertheless, the mechanism whereby resist- 

 ant mutants arise spontaneously and are then 

 selected by the drug can account for every 

 well-studied case of inherited resistance (10, 

 5). Furthermore, in favorable instances the 

 spontaneous origin of drug-resistant mutants 

 can be verified unambiguously by contriving 

 to isolate them without their ever being ex- 

 posed to the drug. One method entails indi- 

 rect selection. To illustrate its application, con- 

 sider a culture of Escherichia coli containing 

 io 9 bacteria per ml. By plating samples on 

 agar containing streptomycin, we infer that 

 one bacterium per million or io 3 per ml pro- 

 duce resistant clones. But to count these clones 

 they were selected in the presence of strepto- 

 mycin which hypothetically might have in- 

 duced the resistance. We may however dilute 

 the original bacteria in plain broth to give 

 samples containing io 5 per ml. Since io~ 6 of the 

 bacteria are resistant, each sample has a mathe- 

 matical expectation of 0. 1 of including a resist- 

 ant bacterium. The individual bacterium be- 

 ing indivisible by dilution, nine samples in ten 

 will include no resistants; the tenth will have 

 one, but now augmented to io" 5 . Which one 

 this is can be readily determined by retrospec- 

 tive assay on the incubated samples. The pro- 

 cedure can be reiterated to enrich for the resist- 

 ant organisms until they are obtained in pure 

 culture (11). The same result is reached more 

 conveniently if we spread the original culture 

 out on a nutrient agar plate rather than dis- 



102 



