Lederberg • Genetic Transduction 261 



leadership of the late O. T. Avery, been estaWished to the limits of cur- 

 Their extensive researches culminated rent anahtical methods. . . . 

 in the report in 1943, that the active Tlie success of Griffith and Avery 

 material in the ' pneumococcus trans- has, of course, inspired many investi- 

 formation" was a macromolecular form gators to look for genetic effects of 

 of desoxvribonuclcic acid, DNA, a sub- DNA with other bacteria. The greatest 

 stance that had alrcad\- been found to success to date has been met with in 

 occur characteristicalh- m the nuclei the influenza bacillus {Hemophilus in- 

 and chromosomes of the cells of higher fluenzae) where again a capsular anti- 

 organisms, and was therefore believed gen and streptomycin resistance proved 

 to be intimately connected with the to be useful markers. This system is 

 stuff of heredity. Griffith had been closely parallel to the pneumococcus 

 rather lucky in his earlier experimental transformation, a result which is im- 

 design. The living mouse proved to portant, in itself, in confirming the 

 have two functions: a selective action, generality of the potency of DNA in 

 as alreadv mentioned, and to furnish transducing activity. On the other 

 protein adjuvants from its serum that hand, many trials with other bacteria 

 play an obscure part in conditioning have been definitely unsuccessful, 

 the bacteria to enable them to take up though negative results do not, of 

 the DNA particles. course, receive equal attention. 



Gonsiderable progress has been 

 made recently in the chemical analysis transduction by phage 



of DNA, and it is known to consist of 



extended chains of nucleotide units. A second mode of transduction, 



Each unit contains phosphoric acid; a mediated by bacterial viruses rather 



five-carbon sugar (desoxyribose); and than chemically extracted DNA, was 



a purine or pyrimidine base (adenine, discovered independently of the pneu- 



guanine, thvmine, or uracil). The mococcus researches in the course of 



specificit^• of DNA is believed to de- studies on Salmonella. This group of 



pend on the sequence of these four al- bacteria is of medical interest because 



ternatives; it is not surprising that very of their connection with diseases such 



large molecules are required to store as food-poisoning and enteric and ty- 



biological information of ultimate com- phoid fever. However, they are closely 



plexity in a language with such a sim- related to Escherichia coli, a species 



pie alphabet. Actually, the true molec- which has been shown to undergo re- 



ular size of pneumococcal DNA is not combination by a sexual mechanism, 



accurately known, for such long linear that is one that involves intact cells and 



macromolecules are the most difficult the exchange of large blocks of genes, 



to study by the ph)sicochemical meth- Originally, sexuality could be demon- 



ods that have been developed for the strated in only one bacterial strain 



proteins. It is generally accepted, how- (K-12) but after suitable methods were 



ever, that the material extracted from developed, the same process could be 



the pneumococci is a mixture of active found in many different strains of E. 



and inactive material, and that the coli. It was therefore natural to extend 



active constituents would consist of these researches to other related orga- 



chains with hundreds or thousands of nisms, of which Salmonella was a 



nucleotide units. The same difficulties choice. 



have stood in the way of rigorous proof The principal criterion for sexual 



that only DNA is involved in the trans- rather than transductive recombination 



forming substances, though this has is the concurrent exchange of several 



