238 



CYTOLOGY 



G. W. Beadle 



What is a Gene? 



Reprinted with the permission of the author 

 and publisher from the A.I.B.S. Bulletin 

 5(5):15, 1955. 



A series of recent developments in 

 biology and chemistry has greatly in- 

 creased our understanding of the struc- 

 ture, function, replication and muta- 

 tion of genetic material. 



Beginning with the work of Avery 

 and his collaborators on pneumococcal 

 transforming principles, evidence that 

 the primary genetic information is car- 

 ried in the form of desoxyribonucleic 

 acid (DNA) has become quite strong. 



The most compelling of the several 

 arguments for assigning this impor- 

 tant role to DNA comes from the ex- 

 periments of Hershey and co-workers 

 (Cold Spring Harbor Symp. Quant. 

 Biol. 18:135, 1953) with bactenal vi- 

 ruses in which the protein coat is 

 labelled with sulfur- 3 5 or the DNA 

 core labelled with phosphorous-32. Tlie 

 course followed by these radioactive 

 tracers through the virus life cycle in- 

 dicates that only the DNA enters the 

 host cell. Since genetic studies show 

 quite clearly that the virus has linearly 

 arranged hereditary material that be- 

 haves in several respects like that of 

 higher organisms, the indicated conclu- 

 sion is that this material in the virus 

 must consist solely of DNA. 



Perhaps of greater significance to 

 biology than any advance of recent 

 decades is the proposal of a specific 

 structure of DNA by Watson and 

 Crick. (Cold Spring Harbor Svmp. 

 Quant. Biol. 18: 123, 1953.) This struc- 

 ture, which consists essentially of a 

 pair of complementary polynucleotide 

 chains hydrogen bonded together 

 through purine and pyrimidine bases, 

 provides a basis for genetic specificity 

 in the order of base-pairs along the 

 molecule. Separation of the comple- 

 mentary chains, followed by directed 

 synthesis of new complementary part- 

 ners by the single chains, is presumed 

 to be the basis of replication of the 

 genetic material. Mistakes in nucleo- 

 tide sequence, made during replication, 

 provide one plausible basis for gene 

 mutation. 



It has been suggested that gene 

 function involves a transfer of DNA 

 information or specificity to ribonucleic 

 acid (RNA) and thence to such macro- 

 molecules as proteins. Since no struc- 

 ture of RNA comparable in plausibil- 

 ity to the Watson-Crick DNA struc- 

 ture has been proposed, the nature of 

 the transfer of DNA specificity to RNA 



