BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 451 



evidence that they function cytologically^''*-^^'' or genetically as sites of 

 mutation^"^ as do nuclei of the higher forms. Furthermore, since bacteria 

 multiply by mother-daughter division, a simple hybridization technique 

 is not available for testing the unit-particle transmissibility of a mutation. 

 A somewhat analogous process, recombination in the K-12 strain of E. 

 coll, has been used for this purpose in the very important work initiated 

 by Lederberg,^''^ but unfortunately the crossing between bacterial cells is a 

 very rare event in these strains and has not yet been produced at all in 

 most strains and species of bacteria. In any case, a host of discrete, dis- 

 continuous, heritable alterations in properties have been observed in 

 bacteria and are now generally designated mutations. Among those in 

 which the all-or-none spontaneous nature of the change has been demon- 

 strated are virus resistance, drug resistance, and various biochemical de- 

 ficiencies. 



(3) Transformations as Stepwise Transfer cf a Mvtaticn Pattern. If we 

 compare the manner in which bacterial mutant characters are acquired 

 spontaneously, and that in which they are transferred by DNA, we find a 

 number ^of parallels. First, when independent properties such as capsule 

 synthesis and penicillin or streptomycin resistance are both being trans- 

 ferred from a single DNA preparation of a doubly marked strain, only 

 one of these properties as a rule is acquired at a time by an individual 

 transformed cell.^^''^'*^ This and other examples appear to reflect the 

 independent existence of different DNA particles determining different 

 unit properties. These observations fit well with the fact that the bacterial 

 strain furnishing the DNA has originally acquired these mutations sepa- 

 rately in single steps and is presumed therefore to have unit genes corre- 

 sponding to these very transforming units detected. 



An analysis of the dose-response relation has indicated that the collision 

 effective in transforming one cell in a pneumococcal microcolony requires 

 only one particle of transforming agent. ^i° Other recent data have shown 

 that one single brief exposure to transforming agent can in certain instances 

 introduce into an individual cell two factors if they both are present in 

 the donor strain from which DNA was prepared .^^ This finding appears to 

 indicate that certain pairs of transforming agents stand in special relation 

 to each other, either chemically within one and the same DNA particle, or 

 perhaps in respect to the way they can be accepted by a single cell. 



A striking parallelism between the transforming factor and the hypo- 



»«« K. A. Bisset, Cold Spring Harbor Symposia Quant. Biol. 16, 373 (1951). 



1" B. Delaporte, Advances in Genet. 3, 1 (1950). 



i»8 E. M. Witkin, Cold Spring Harbor Symposia Quant. Biol. 16, 357 (1951). 



109 J. Lederberg, Genetics 32, 505 (1947). 



"» A. W. Ravin, Exptl. Cell Research 7, 58 (1954). 



