254 Structure of Viruses / 1 4 : 4 



4. Physical Biochemistry of Viruses 



Chemical analyses of the T series of coliphage show that those of this 

 type consist of two classes of compounds : proteins and nucleic acids. 

 Both are condensation type polymers ; their chemical structure and form 

 are discussed further in the. next chapter. For the present, it is sufficient 

 to note that proteins form part of the cell membranes and also part of all 

 enzymes (that is, substances controlling the rate of biological reactions) . 

 The other class of compounds in T phages, the nucleic acids, is concerned 

 with the transmission of genetic information and the synthesis of pro- 

 teins. Two types of nucleic acids are known : DNA and RNA (Deoxy- 

 ribose Nucleic Acids and Ribose Nucleic Acids). DNA is associated 

 with genetic information in plants, animals, and bacteria, whereas both 

 types appear to be associated with protein synthesis. 



The bacteriophages all contain a large amount of DNA. This exists 

 as a core inside a protein layer. The structure of some plant viruses is 

 similar except that they contain RNA in place of DNA. The structure 

 of some animal viruses is more complex, but all contain nucleic acid. 

 The action of the bacteriophages has received more detailed study and 

 will be discussed in the remainder of the section. The life cycle of a 

 T phage is represented in Figure 5. 



The T phages all attach to the bacterial surface by their tails. This 

 attachment is at first reversible, but then certain enzymes, presumably 

 proteins on the tip of the tail, make it irreversible. Certain receptor 

 sites appear necessary for phage attachment. If the nucleic acid is 

 removed from the phage (which can be done in the case of the even 

 numbered T phages by osmotic shock) the phage particles attach to the 

 bacterial surface exactly as if they were whole, but fail to reproduce. 

 If an excess number of phage particles attack one bacterium, the cell 

 undergoes "snap lysis"; that is, it breaks without reproducing phages. 

 This also occurs when bacteriophages without nucleic acid are used. 



After the complete bacteriophage attaches to the cell wall, it empties 

 its nucleic acid content but none (or almost none) of its protein into the 

 cell. The protein phage ghosts can be removed mechanically from out- 

 side the bacteria without interfering with phage reproduction. In 

 contrast, if phages are mixed with broken pieces of bacterial cell walls, 

 they attach to these pieces, emptying their nucleic acid content out 

 through the other side of the cell wall. 



Once the phage nucleic acid is inside the bacterium, it alters the 

 metabolic processes of the bacterial cell. In some cases, the cell may 

 divide for several generations carrying the phage with it in a latent form 

 called a prophage. The cell is said to be in a lysogenic state. Eventually, 

 the prophage is induced to enter the active phase called vegetative. The 



