256 Structure of Viruses / 14 : 5 



two bacteria join together, some of the DNA from one passing into the 

 other. The one receiving the DNA takes on new characteristics typical 

 of the donor. These may include resistance to antibiotics, clone shape 

 and size, form of cellular wall, and metabolic nutrients required. 

 Essentially, the same results can be obtained by exposing the bacteria to 

 high concentrations of DNA extracted from a strain having slightly 

 different characteristics. The DNA molecules apparently pass through 

 the cell membrane and alter the genetic properties of the cell. 



Infection by bacteriophage is an extreme example of adding foreign 

 DNA (the result of which is the acquirement of new properties which 

 are fatal to the cell) . During the formation of new phage particles, the 

 nucleic acid threads appear to break and then recombine, not always 

 with the same partners, but always with partners of the same length. If 

 a single cell is infected with several strains of the same type of phage, 

 this recombination can lead to new phages having some properties of 

 each of the parent strains. This makes it possible to study phage 

 genetics. (The experimental evidence for recombination does not 

 necessarily imply that the nucleic acid thread actually breaks. Many 

 other models of DNA replication also include the possibility of recom- 

 bination.) 



5. Phage Genetics 



The techniques of recombination between phage strains have been used 

 to study the genetic fine structure of the E. coli phage T4. As was 

 mentioned in Section 2, different genetic characteristics of phage strains 

 have been described by such factors as plaque shape, strains of bacteria 

 infected, rate of lysis, formation of lysogens, and details of the shape of 

 the mature phage particles. The relative frequency of the appearance 

 of a new property when two strains of bacteriophage are mixed with the 

 host bacteria is interpreted as the probability of recombination. If 

 both strains completely lack one property, such as the ability to form 

 plaques on a given strain of bacteria, then it is comparatively easy to 

 measure the occurrence of this property when the two phage strains are 

 mixed. The probabilities of recombination between two such strains to 

 form phage particles when the property is lacking in both parent strains 

 is a measure of the distance between the locations of the two mutations 

 along the DNA chain of the bacteriophage. In terms of these recom- 

 bination probabilities, three types of units have been defined for mapping 

 the genetic properties of the DNA of the bacteriophage. These three 

 units, the cistron, the recon, and the muton, were introduced in Chapter 

 10; they are redefined in this section in terms of the properties of the T4 



