Organization and Replication of DNA in Vivo 



269 



stabilizing bond holding the members of a 

 base pair together is called a hydrogen bond 

 or H bond. The base pairs, their H bonds 

 indicated by interrupted lines, are shown in 

 Figure 20-5; the hydrogens that are re- 

 moved when the base pairs join the back- 

 bones are included in the diagrams. The 

 top half of the illustration shows the C-G 

 (and G-C) arrangements. Note in the C-G 

 pair that cytosine has been turned over (from 

 left to right) relative to the way it was 

 shown in Figure 19-3. Three H bonds are 

 formed. Two occur between NFL. and O 

 (the 6— NH, of C with the 6— O of G; 

 the 2— O of C with the 2— NFF of G). 

 One occurs between the 1 — N of C and the 

 1 — NH of G. The GjC pair is identical to 

 C;G shown except that, in this case, the 

 base turned over is guanine. 



The bottom half of Figure 20-5 shows 

 the other type of base pair (T:A or A:T, 

 in which T and A have been turned over 

 relative to the way they were shown in Fig- 

 ures 19-3 and 19-4). In this pair only 

 two H bonds are formed, one between the 

 6 — O of T and the 6— NH,. of A; the other 

 between the 1 — NH of T and the 1 — N of 

 A. Although the hydrogen bond is a weak 

 chemical bond as compared to the C — C 

 bond, there are so many H bonds along a 

 long double helix that the entire structure 

 is fairly rigid and paracrystalline even when 

 moderately hydrated. Note that the region 

 surrounding two base-paired nucleosides 

 can be separated into two portions relative 

 to the pentoses. The smaller portion is 

 called the minor groove (the region sur- 

 rounding the lower parts of the base pairs 

 shown in Figure 20-5), and the larger por- 

 tion, the major groove (the region surround- 

 ing the upper parts). 



Recall that the double helix configuration 

 of DNA does not dictate the sequence of 

 bases along the length of a chain. But re- 

 member also that the sizes of the pyrimidines 

 and purines and their H bonds do dictate 



Cytosine 



Guanine 



H 



--H-N 



/ 



Guanine 



Cytosine 



CH- 



O 



N-H 



\ i>— - u // \\ 



H 7 > 



Thymine 



Adenine 



O H 



Thymine 



FIGURE 20-5. 



Base pairs formed between single DNA strands. 



