Viruses — Cell Parasites 73 



to be drawn from this is that bacteriophages break up in the cell into 

 smaller particles which multiply independently and reassemble so as 

 to give rise to all their possible combinations in the new individuals. 

 Thus we find a kind of sexual reproduction even below the level of 

 cells. 



Another experiment by S. E. Luria of Indiana University also 

 indicates the composite nature of bacteriophages. Viruses can be 

 'killed' by exposure to ultra-violet light. When infected by a single 

 'killed' bacteriophage particle the bacterium usually dies, but no mul- 

 tiplication of the virus takes place; but if two or more of the 'killed' 

 virus particles enter the bacterium, it was found that they often 

 multiplied as well as the undamaged virus. It looks as if the two 

 damaged particles can share their parts and make up one complete 

 individual between them. Luria estimated from his results that the 

 'phage particle has at least twenty independent units. When only one 

 of these is damaged by ultra-violet light, the particle is incapable of 

 multiplication, but the damaged unit can be made good from other 

 particles. 



This reconstitution of bacteriophages from damaged parents has 

 been carried very much further by S. Benzer. By treatment with ultra- 

 violet light or by ionizing radiations, it is possible to obtain a con- 

 siderable number of mutants of a bacteriophage, which can be de- 

 tected by their effects on their original host cell; or on slightly different 

 strains of the bacterium, which react in different ways to infection. 

 It is possible also to study the recombination of these different forms 

 so as to reconstitute the original bacteriophage. 



In these experiments it has been possible to construct a 'map' 

 showing the order in which the different genes occur. This must mean 

 that although recombination of the genes derived from two different 

 'phages can occur, it cannot be a haphazard process. That is, we 

 must not think of the bacteriophage being dissolved into its com- 

 ponent units which are then reassembled in a haphazard way. 

 Although recombination of the parts occurs, it must be carried out 

 in such a way that the order in which the genes are arranged is not 

 lost. We must picture the bacteriophage particles as exchanging parts 

 with each other, without losing their overall structure. This is similar 

 to what happens in chromosomes, where as we have seen, in the 

 formation of the germ cells, two chromosomes line up near each 

 other and exchange. But the bacteriophage particle is much smaller 

 than the chromosome and in fact it has been found that forty per 

 cent of the dna inside it is a single dna particle, having a molecular 

 weight probably of about fifteen million. The fact that exchange of 

 genes between two bacteriophages can occur, means that a single 



