BIOLOGICAL ORDER 



each virion possesses an icosahedral symmetry. It is built up out of 

 60 structurally equivalent asymmetric units of approximately 60 A 

 diameter. If 60 protein subunits of that size are packed together in 

 close contact in a way consistent with icosahedral symmetry, they 

 form a roughly spherical shell of about 300 A diameter. 



This type of structure is clearly seen in electromicroscopic photo- 

 graphs of many viral species. The protein coat of viruses or capsid 

 is actually made up of subunits or capsomeres. 



The molecular weight of the virion is approximately 6.7 X lO^*, 

 of which the protein represents about 62%, the nucleic acid 28%. 

 The protein of the capsid contains about 49,600 amino acids. The 

 nucleic acid is composed of about 5,200 nucleotides. 



In the discussion of the code, the conclusion was reached that a 

 sequence of three nucleotides was probably necessary to take care 

 of one amino acid. Therefore, the maximum number of amino 

 acids that can be organized in the viral nucleic acid is 1,700. The 

 genetic material of the virion therefore cannot directly build a 

 molecule composed of 49,600 amino acids, but it can build a number 

 of smaller molecules. Each capsomere is in fact made up of approxi- 

 mately 620 amino acids. The capsid can only be built of subunits, 

 and is in fact built of subunits. 



The structure of the viral infectious particle is extraordinary 

 in the sense that it is unique. No other "biological" particle or 

 organelle is yet known which is composed of a large number of 

 asymmetrical subunits exhibiting relations of symmetry. This type 

 of structure seems to be able to form only a viral coat and nothing 

 else. 



An entity containing a small number of protein species, let us 

 say one to five, is unable to do much work. It cannot secure energy 

 and cannot synthesize its building blocks. Even if some of the pro- 

 teins are endowed with a potential enzymatic activity, the virion is 

 metabolically inert. The viral genetic material, in order to express 

 its potentialities, that is, to synthesize the specific viral proteins and 

 to duplicate itself, must necessarily be in an environment where 

 building blocks and energy are provided. These conditions exist 

 only inside a cell, which is the critical living mass. 



The essential function of the protein viral coat is to provide a 

 means for the transmission of the genetic material from cell to cell 



[64] 



