6 MOLECULES, VIRUSES, AND BACTERL^ 



the order of the monomers appears to be very precisely determined, 

 and we have some grounds for beHeving that this is true for the nucleic 

 acids. Thus if we concentrate on the chemical bonds of the macromole- 

 cules, we see that their "structure" is like that of a linear language. 



What I want to consider here, however, is not the chemical bond- 

 ing (their "primary structure") but the secondary and tertiary struc- 

 ture due to weaker bonds, such as hydrogen bonds, salt linkages, van 

 der Waal forces, etc. In short, how they fold up and how the ways they 

 can fold are related to their function. 



Let us first consider the nucleic acids. We know rather little about 

 the structure of RNA, but we have as evidence the double helix of 

 DNA and the structure of the RNA-like polymers— the synthetic poly- 

 ribotides— such as polyadenylic and polyuridylic acids, etc. An early re- 

 view of the evidence is given in Crick ( 1957 ) and a more recent one by 

 Rich (1959). 



The remarkable fact emerges that so far there is no simple regular 

 structure for nucleic acid having onhj one chain. We can have regular 

 helices with two chains or with three chains wound helically around 

 one another ( so far none is known for certain to have four chains ) . Of 

 course, a single chain might take up a "two-chain" type of structure by 

 folding back on itself, but for the moment I am excluding such compli- 

 cations. Moreover, in all these structures, as far as we know, the chains 

 are held together merely by weak bonds between bases on diflFerent 

 chains. 



When we come to consider proteins and synthetic polypeptides, 

 we find just the opposite picture. No simple two-chain structure is 

 known. We have one three-chain structure— that of collagen— but the 

 chains are not held together very firmly, and there is some restriction on 

 the amino-acid sequence ( every third one must be glycine ) . In particu- 

 lar, interactions between the backbones are what mainly hold the 

 structure together, though the interaction between pyrrolidine rings 

 appears to help. It seems unlikely that the structure of collagen could 

 ever be the basis for a simple, precise replication mechanism, based on 

 the pairing of side-chains. 



What we do find for polypeptides is that the simplest regular struc- 

 ture is of a single chain coiled helically on itself— the well-known alpha- 

 helix— and we now have no doubt, thanks mainly to the work of Ken- 

 drew and his colleagues ( 1960 ) , that the alpha-helix is important in 

 globular proteins. 



The significant point about the alpha-helix, however, appears to be 

 its stability. The backbone itself is, in a loose sense, only marginally 

 stable in water. That is, many sequences of side-chains will fold up 

 into an alpha-helix, but it is possible, by the interaction of side-chains, 



