50 Nature of the Genetic Material 



present materials and are not just added to the existing material (i.e., 

 they are incorporated into a chromosome). There are also instances, 

 in both transformation and transduction, in which linked properties 

 are transferred together. In transformation, then, the genetic material 

 is isolated by the extraction process and enters directly the bacterium 

 which it transforms. In transduction the transferable material is iso- 

 lated by partial lysis due to phage, is then incorporated into or 

 attached to the phage, and is transduced with it. Since closely hnked 

 properties, involving related reactions, are transferred in both cases, 

 Zinder is ready to compare this situation to pseudoallelic conditions 

 in Drosophila. 



The facts and interpretations amount to the claim that genes 

 or genie material are transduced; it might then be asserted that 

 the DNA in the pneumococcus transformation is a piece of genie 

 material, assuming that the method of its extraction excludes the 

 presence of attached protein. The reported facts may be described 

 as the replacement of the nucleic acid component of a genetic struc- 

 ture in the cell (because of its mutability) by an introduced, external, 

 and different though closely related nucleic acid, extracted from a 

 different organism for which it is characteristic. It is reported as 

 certain, so far as experimentation goes, that the transforming agent 

 is a pure polymerized DNA, although different kinds of molecules of 

 DNA are present in the extracts, and it is rather tempting to con- 

 sider them to be genes. Hotchldss and Marmur (1954) found recently 

 that in bacterial transformation one DNA extract could transfer two 

 different genetic properties simultaneously. This is compared directly 

 with linked genes. If these conclusions were unavoidable, the nucleic 

 acid nature of the genetic material would be proved. 



But it is far from certain that the interpretation is so simple, 

 apart from the variations of the results reported above. All modern 

 information tends to show that bacteria have regular chromosomes 

 ( Robinow, De Lamater; see De Lamater, 1951 ) and that crossing over 

 occurs according to the classical scheme (Lederberg and Tatum, and 

 others). If this is true also for the organisms in which transformation 

 has been found (actually E. coli is one of them), and if the transform- 

 ing agent is genie, transformation should be described thus: the 

 DNA extracts used for transforming contain molecules of different 

 specific DNA's, and are of the nature of genes. These are taken into 

 the treated bacterium, where they push out of the chromosomes the 

 homologous gene present and assume its place for good; or, what 

 amounts to the same, they force the homologous gene to change its 



