250 



Structure of Viruses / 1 4 : 2. 



the original one, thereby producing a pattern which is characteristic of the 

 particular phage. These clear spaces are called plaques. Figure 3 

 shows typical plaques for two strains of T2 bacteriophages infecting 

 E. coli. Note that in Figure 2 the clones are bacteria on a clear back- 

 ground, whereas in Figure 3 the plaques are clear spots in a uniform 

 layer of bacteria. 



This type of experiment and others 

 using isotopic tracers have developed 

 the following picture of bacteriophage 

 activity. Outside the cell, the bac- 

 teriophage is initially attached revers- 

 ibly to the bacterial cell wall. Once 

 in contact, it opens a small hole 

 through the cell wall and dumps the 

 central part of the phage particle into 

 the bacterium. Inside, the phage 

 apparently breaks into smaller pieces. 

 This can be shown by rupturing the 

 bacteria sonically, after the phage has 

 just entered. No active particles are 

 found on plating. 



Immediately upon entering the cell, 

 the pieces of bacteriophage take over 

 control of the cellular metabolism, 

 directing it toward the production of 

 many new phage particles. This period 

 is called the induction period or the eclipse. 

 Eventually (that is, after about 10 minutes at 37°C), the new phage 

 particles start forming. When about a hundred of these are completed, 

 the bacterium bursts, releasing the new phage particles into its sur- 

 roundings. (This bursting is called lysis.) The new particles produced 

 are replicas of the original one. If other bacteria are present, the cycle 

 repeats; otherwise, the phage particles remain in suspension, not meta- 

 bolizing or behaving in any fashion as living matter. 



Because the phage particles are very small compared to the bacteria, 

 more than one may enter a bacterium. If different types of phages are 

 mixed, certain ones are able to "cross breed"; that is, some of the new 

 phage particles have characteristics in between those of the two original 

 strains. If two damaged phage suspensions are mixed, in some cases 

 active phage can be produced by this cross-breeding process. In other 

 words, genetic recombination has occurred. 



It is also possible for a phage particle occasionally to change its 

 characteristics, apparently spontaneously. (The characteristics include 



Figure 3. Phage plaques. This fig- 

 ure shows T2 plaques formed on E. 

 coli B. The smaller plaques are wild 

 type, whereas the larger ones are r- 

 mutants. 



