THE STRUCTURE OF DNA 



ADENINE ° THYMINE 



4\ 



5 ii 



I I I 



Figure 5. Pairing of adenine and thymine. Hydrogen bonds are shown dotted. 

 One carbon atom of each sugar is shown. 



CUANIM 



CYTOSINE 



Figure 6. Pairing of guanine and cytosine. Hydrogen bonds are shown dotted. 

 One carbon atom of each sugar is shown. 



The essential element of the structure is the manner in which the two chains 

 are held together by hydrogen bonds between the bases. The bases are perpendic- 

 ular to the fiber axis and joined together in pairs. The pairing arrangement is 

 very specific, and only certain pairs of bases will fit into the structure. The basic 

 reason for this is that we have assumed that the backbone of each polynucleotide 



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