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X-ray Analyses of Proteins and Nucleic Acids /1 5 : 6 



C 



P 



O o 



O H 



Base pairs 



with about 10 rungs per turn. The 

 pairs of bases fit across the chain as 

 rungs being supported in the middle by 

 hydrogen bonds. It is necessary that 

 each pair of bases fit very exactly. 

 Measurements based on X-ray diffrac- 

 tion patterns of crystals of the purine 

 and pyrimidine bases have shown that 

 these do indeed fit, provided that 

 adenine (A) is paired with thymine 

 (T), and guanine (G) with cytosine 

 (C). If this is the case, one should 

 have the relative concentrations of 

 organic bases in DNA related as 



[A]/[T] = [G]/[C] = 1.0 



This relationship had been verified for 

 all DNA and was one of the pieces of 

 evidence used by Crick and Watson to 

 construct their model. 



Gamow tried to explain protein for- 

 mation in terms of the location of 

 residues along the DNA chain. The 

 diagram in Figure 14 shows the type 

 of blocks Gamow considered to deter- 

 mine amino acid arrangement. In the 

 outlined cross, G, T, and C are in- 

 dependent, but the A is determined by 



Figure 13. The helix of DNA, with three 

 different ways of representing the molecular 

 arrangement. Top, general picture of the 

 double helix, with the phosphate-sugar com- 

 binations making up the outside spirals and 

 the base pairs the cross-bars; middle, a some- 

 what more detailed representations: phos- 

 phate (P), sugar (S), adenine (A), thymine 

 (T), guanine (G), cytosine (C), and hydrogen 

 (H) ; bottom, detailed structure showing how 

 the space is filled with atoms: carbon (C), 

 oxygen (O), hydrogen (H), phosphorus (P) 

 and the base pairs. After C. P. Swanson, 

 The Cell, (Englewood Cliffs, N. J. : Prentice- 

 Hall, Inc., 1960). 



