114 CELL HEREDITY 



duction of diploid zygote nuclei which unite two complete genotypes. 

 In this regard, and because meiosis sooner or later ensues, syngamy and 

 sexual reproduction are similar. 



With the advent of more extensive investigations into the genetics of 

 microbes, other mechanisms of mating ha\'e been revealed. Some in- 

 \'()l\e the incomplete transfer of the genotype of the donor individual, 

 and the recipient has more than one, but less than two, sets of genes. 

 In such partial zygotes meiosis, and even mitosis, may not occur; the 

 partitioning of genetic material is apparently some simpler consequence 

 of the replication of DNA. 



MATING AMONG VIRUSES 



Even viruses are able to exchange their genes. To ask whether these 

 minute structures are living or not is a question of semantics. What- 

 ever the answer, when they are inside host cells they behave like many 

 other cell particulates, influence the biochemistry of the host, and multi- 

 ply. Such extensive information about the biochemistry of viral infection 

 has been obtained that it has been possible to relate genetic to chemical 

 events. The term molecular biology is not inappropriate when applied to 

 many investigations in this field. 



For reasons of experimental convenience, the most detailed studies of 

 host-virus relations have been conducted with bacterial viruses, notably 

 the T phages which attack E. coli B (see Chapter 1 ). Thev are called 

 bacteriophages because they destroy their bacterial host. When a dilute 

 phage suspension is spread on a thin film of sensitive bacteria growing 

 on an agar plate, each phage particle will adsorb on one bacterium and 

 multiply within it until the cell breaks down (lyses) and releases newly 

 formed phages. These, in turn, infect adjacent cells, and the process 

 continues until a visible area on the plate has been cleared of bacteria. 

 Such a spot is called a plaque (Figure 3.8), which is the usual unit of 

 observation in experiments with phages. The appearance of the plaque 

 (turbiditv, margin, etc. ), and its size depend upon phage genes which 

 can mutate. 



When a virulent phage adsorbs on the surface of a sensitive bacterium, 

 the synthetic reactions of the host cell which were going on before in- 

 fection are blocked; respiratory metabolism is arrested in the state exist- 

 ing at that moment, and it is no longer possible to elicit the formation 

 of adaptive enzymes. The formation of new protein begins and there 

 is a turnover of RNA which, as it is degraded, is replaced by a new type. 

 DNA synthesis ceases completely (Figure 5.1 ). As a matter of fact, the 



