310 



CHAPTER 34 



H 



\ 



N-H^.- 



O 



h^/ ^-^ \y_Ti 



Cytosine 



\ 

 W Guanine 



H 



..-^H-N 



/ 



/ 





-N 



H 



Guanine 



H 



Cytosine 





N-H^.. 



H 



H 





Thymine 



Adenine 



^H— O^ ^"' 



H 



-N 



-N 



O H 



Thymine 



FIGURE 34-4. 



Base pairs formed between single DNA chains. 



are so many of them along a long double 

 helix that the entire structure is fairly rigid 

 and paracrystalline even when moderately 

 hydrated. 



You will recall that the double helix con- 

 figuration of DNA does not dictate the 

 sequence of bases along the length of a chain. 

 But you will also remember that the sizes of, 

 and the H bonds in, the pyrimidines and 

 purines did dictate that A in one chain can 

 pair only with T in the other chain, and C with 

 G, in order to form a double helix of constant 

 diameter whose strands are held together by 

 the maximum number of H bonds. Since A 

 and T always go together, as do C and G, the 

 equivalences A = T and C = G, found when 

 DNA is analyzed chemically, become mean- 

 ingful as being the direct consequence of the 

 secondary structure of DNA. In fact, the 

 chemical equivalences provide the first inde- 

 pendent test of the Watson-Crick model, 

 which was constructed initially on the basis 

 of other considerations. 



Recall that in order to maximally H-bond 

 a purine and a pyrimidine, it was necessary 

 to represent one of the two as being turned 

 over, so that the number I atoms of both face 

 each other. This has an important conse- 

 quence for the orientation of the two chains 

 relative to each other, as is illustrated by 

 means of Figure 34-5. The bases in the 

 chain at the right all face the accustomed way, 

 while those in the left chain are all turned 

 over. In order that each base join to its 

 sugar in the same way, the sugars must be 

 arranged as shown. Notice, in proceeding 

 downward from the top of the right chain, 

 that the PO4 linkages to sugar read 3'5', 3'5', 

 etc. But, when read the same way, the left 

 chain is 5'3', 5'3', etc., so that the member 

 chains in a double helix run in opposite 

 directions, as indicated by the arrows. 



The X-ray diffraction results, which led to 

 the double hehx hypothesis, do not tell us 

 that all DNA in chromosomes is two- 

 stranded, or that a double strand is never 



