Genetic Systems I ' 163 



absence of sexual reproduction, recombination of the genetic ma- 

 terial can occur during the synthetic processes producing more DNA. 



Transduction 



The bacteriophages discussed previously are virulent, and the host 

 bacterium is killed as the cell undergoes lysis. However, there are 

 also temperate phages capable of establishing a sort of symbiotic 

 relationship which need not result in lysis of the host bacteria. 

 These are called lysogenic bacteria (for example. Salmonella). The 

 noninfectious stage of the temperate phage is called prophage. Re- 

 production of the prophage and bacterium is so regulated and inte- 

 grated that there is no detrimental effect. Occasionally, however, 

 at a rate of 10 "- to lO"-" per generation, lysis of a bacterial cell 

 occurs and the phages released are able to adsorb on other bacterial 

 cells. 



In the course of DNA synthesis, the prophage may incorporate 

 some of the genetic material of the host bacterium. If the prophage 

 should convert to the phage stage, the host will lyse. When the 

 phage is released and infects a new bacterium, it carries with it 

 genetic material from the former host. This in turn has a chance of 

 being incorporated into the genotype of the recipient bacterium, a 

 phenomenon known as transduction. The genetic material trans- 

 ferred usually is not a single genetic factor but groups of factors. 

 Transduction thus is a special sort of genetic recombination, again 

 involving not the sexual process but an infectious process. The 

 phage acts as a vector for infectious transfer of bacterial genes. 

 Viruses, including phages, with RNA are also known, but their study 

 is just beginning. 



Sexual Recombination in Bacteria 



Sexual recombination also occurs in bacteria. Tatum, Lederberg, 

 and others have shown that in Escherichia coli conjugation of cells 

 may take place. Genetic material is transferred from one cell to an- 

 other, one of the conjugating cells being a recipient cell, the other 

 a donor. Recombination of traits from diflFerent strains has been 

 studied in some detail. The process of conjugation can be inter- 

 rupted by separating the cells by agitation, and it can be shown that 

 the amount of genetic material transferred is proportional to time. 

 The genetic material of the bacterium behaves as if it were on a 

 single chromosome. Linkage maps of this chromosome have been 



