Genes: Nature and Mode of Action - 521 



and R. Hotchkiss working in the United 

 States. 



Bacterial Transformation. Previous to the 

 discovery of bacterial transformation, it was 

 generally believed that the only manner in 

 which genes could be passed on from genera- 

 tion to generation was by way of chromo- 

 somes, transmitted by the complex mecha- 

 nisms of mitosis, meiosis, and fertilization. 

 The transformation experiments, on the other 

 hand, led inevitably to the conclusion that 

 certain bacterial genes can be passed from 

 cell to cell through the culture medium in 

 which the bacteria are growing, and that the 

 transmitting agency must be DNA. 



Some 30 different bacterial transforma- 

 tions have now been studied. Here, however, 

 only one will be described. This deals with 

 the inheritance of a protective capsule in 

 Diplococcus pneumoniae, the pathogenic 

 agent that causes bacterial pneumonia in 

 man. 



Two strains of this bacterium can be dis- 

 tinguished — one naked, the other covered by 

 an easily visible capsule. Each of these strains, 

 cultured separately, breeds true; that is, the 

 naked and the covered types each give use 

 only to their own kind. However, if naked 

 cells are exposed to a medium prepared from 

 killed, broken-up, covered cells, quite a num- 

 ber of the previously naked cells now de- 

 velop capsules. Following this transforma- 

 tion, moreover, the newly encapsulated 

 strains retain this characteristic from genera- 

 tion to generation indefinitely. In other 

 words, a heritable change has been stamped 

 upon an organism by material transmitted 

 through the environment. 



Further experiments clearly showed that 

 the bacterial transforming material is always 

 one or more of the high molecular weight 

 DNA compounds, devoid of any protein. In 

 some experiments, two transformations — for 

 example, encapsulation and drug resistance 

 — have been induced to occur simultane- 

 ously; and such changes may display linkage 

 phenomena when their further hereditary 

 transmission is followed. Moreover, the bac- 



teria show waves of susceptibility to trans- 

 formation, which may be correlated with the 

 division cycles of the cells. 



MACROMOLECULAR STRUCTURE OF DNA 



A tremendous impetus to the study of bio- 

 chemical genetics resulted in 1953 from the 

 collaboration of F. H. C. Crick, an English 

 worker, and J. D. Watson, an American. The 

 Watson-Crick model of the molecular struc- 

 ture of DNA, which is shown in Figure 27-1, 

 represented a synthesis of data from many 

 sources; subsequently the essential features 

 of this structure have been validated by many 

 kinds of evidence. Consequently it was not 

 surprising that a Nobel Prize was awarded to 

 Crick and Watson, in 1962. 



Previous analytical work by E. Chargaff 

 at Columbia University, and by Mirsky and 

 co-workers at the Rockefeller Institute, pro- 

 vided helpful preliminary information. Unit 

 for unit adenine and thymine are always 

 equal to each other in any DNA molecule, 

 and the same is true for the other two organic 

 bases, guanine and cytosine (Fig. 27-1). 

 Moreover, the molecular dimensions of DNA 

 structure (Fig. 27-1) were ascertained by the 

 x-ray diffraction studies of M. H. F. Wilkins 

 and others at King's College, London. The 

 Watson-Crick structural model recognized 

 these (and other) features. The analysis 

 showed that only a double helix, such as is 

 shown in Figure 27-1, would fit all the speci- 

 fications. 



The rungs of the coiled ladderlike struc- 

 ture of DNA are constituted by base pairs — 

 cytosine-guanine and adenine-thymine — at- 

 tached to one another by hydrogen bonds 

 (p. 87). The sides of the ladder are formed 

 by two (deoxyribose) sugar-phosphate chains, 

 running in opposite directions, on either 

 side. The space is restricted, however. Only 

 a purine-pyrimidine pair (C — G or G — C or 

 A — T or T — A) can achieve hydrogen bond- 

 ing across the rungs of the ladder (Figs. 27-1 

 and 27-2). 



The length of the double helix of a DNA 



