240 



ter chains. Functional units of DNA 

 in some manner transfer specificity to 

 RNA units which in turn serve as tem- 

 plates in protein synthesis. 



It is conceivable that the transfers 

 of information from DNA to RNA and 

 from RNA to protein are under 

 some circumstances reversible proc- 

 esses. Such reversibility would allow for 

 the storage of genetic information in 

 the form of RNA in those viruses that 

 appear to contain no DNA. Since such 

 viruses replicate their genetic informa- 

 tion only in a host cell containing DNA 

 and protein, it is not necessary to as- 

 sume that RNA is self-replicating in 

 the same sense in which DNA is postu- 

 lated to accomplish this remarkable 

 feat. 



CYTOLOGY 



On this hypothesis, what is a gene? 

 Is it a functional unit carr)'ing the in- 

 formation necessary for the synthesis 

 of a specific macromolcculc? Or is it 

 the unit of recombination? If it is the 

 latter, the evidence from viruses and 

 bacteria suggest that the ultimate unit 

 might well be a single nucleotide. If it 

 were necessary to define a gene with- 

 out additional information, the choice 

 would have to be an arbitrary' one. It 

 may well be that the time is near for a 

 reconsideration of terminology. Until 

 then it is particularly important to 

 make perfectly clear in particular cases 

 just what one has in mind when such 

 terms as gene, locus, and allele are 

 used. 



QUESTIONS 



1. Describe the Watson-Crick model of 

 DNA. 



2. What is the relationship existing be- 

 tween DNA and RNA? 



i 



3. What is transduction? 



4. Would you say that Dr. Beadle is en- 

 tirely satisfied with current definitions 

 of the gene? 



