BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 443 



zation of the DNA. One of the biochemical systems most sensitive to X- 

 rays in tissues appears to be the DNA synthesizing system^"*'** (see Chapter 

 26). 



At the cytogenetic level, the sulfur and nitrogen mustards and X-rays 

 produce a variety of major and minor breakages and rearrangements of 

 the chromosomes, involving loss and refusion of chromosome strands, etc. 

 It may hardly be expected that a single mechanism like the depolymeriza- 

 tion of DNA lies behind all of these effects, but the vulnerability of DNA 

 to these agents is a striking fact that surely has some relation to their 

 mutagenic effects. 



b. Bacterial Transformations — Observed Genetic Action of Isolated DNA 



The cytochemical studies of the DNA content of nuclei and the chemical 

 studies with mutagenic agents were carried out during the last decade, 

 when such studies were stimulated and influenced by the important finding 

 that isolated bacterial DNA can have a genetic effect upon bacterial cells. 



In 1944, Avery et al. reported that DNA extracted from an encapsulated 

 ("smooth"; S) strain of pneumococcus would transform unencapsulated 

 ("rough" colony type; R) cells into encapsulated cells. ^^ There were three 

 especially significant features about these transformations: (1) The capsule 

 of the newly modified cells was always constituted of the same serologically 

 type-specific polysaccharide as that of the strain from which the DNA 

 was derived. (2) The conversion could be brought about in a rough pneu- 

 mococcal strain which was never observed to change or mutate spon- 

 taneously into that smooth type or any other. (3) The smooth cells pro- 

 duced would propagate indefinitely as an encapsulated strain without 

 further exposure to the DNA — in fact, would themselves go on to produce 

 unlimited quantities of DNA with the same potentialities. See Fig. 1. 



It was clear from this work that the pneumococcal DNA preparation 

 had performed two functions usually attributed to genes: it had induced a 

 specific inheritable property (capsule synthesis), and it had initiated its 

 own reduplication (formation of more DNA with the same activity) as 

 well. Although only one single category of character in a single species was 

 involved, the genetic implications of these findings were not overlooked 

 and have had a profound effect in orienting much subsequent research in 

 a number of related fields. The original authors principally concentrated 

 their attention on the nature and properties of the transforming agents 



" J. S. Mitchell, Brit. J. Exptl. Pathol. 23, 309 (1942). 



^^ H. von Euler and G. von Hevesy, Arkiv Kemi 17A, No. 30, 1 (1944). 



56 B. E. Holmes, Brit. J. Radiol. 22, 487 (1949). 



"G. Hevesy, Nature 163, 869 (1949). 



5* R. Abrams, Arch. Biochem. and Biophys. 30, 90 (1951). 



5^ O. T. Avery, C. M. MacLeod, and M. McCarty, J. Exptl. Med. 79, 137 (1944) . 



