10 



CELL HEREDITY 



phosphate 



phosphate 



phosphate 



phosphate 



phosphate 



sugar — adenine •••thymine — sugar 



phosphate 



; sugar — thymine ••• adenine — sugar 



\ 



phosphate 



; sugar — guanine • • • cytosine — sugar' 



;sugar — thymine •• • adenine — sugar 



/ 



phosphate 



; sugar — cytosine • • • guanine — sugar 



/ 



phosphate 



phosphate 



FIGURE 1.2a. Schematic diagram of DNA. The two chains are antiparallel, as 

 shown by the arrows. The dotted lines between the bases represent hydrogen 

 bonding. Although the chains are drawn flat in this diagram, they are actually 

 wound around each other in the molecule, as in Figure 1.2b. 



ization of polynucleotides into a double helix. This helix, shown in 

 Figure 1.2, consists of two chains wound around a central axis, and 

 held together by hydrogen bonds between nitrogen bases which are 

 opposite each other, one on each chain. 



By constructing scale models compatible with the X-ray data, Watson 

 and Crick were able to show that the bases could fit into this double 

 helix only if they were paired in a specific way — adenine with thymine 

 and guanine with cytosine, as shown in Figure 1.3. This specific pairing 

 requirement is entirely consistent with the base ratio data (Table 1.2); 

 DNA molecules do contain precisely the molar ratios demanded by the 

 double helical model. The excellent correspondence between chemical 

 fact and crystallographic possibility has led towards acceptance of the 

 double helix as the native configuration of DNA. 



A most important consequence of the pairing relationship in the 

 double helix is complementarity of the two polynucleotide strands. If 

 the strands become separated, and a new strand is formed in relation to 

 each of the old ones, then each new strand will be identical with the old 

 strand with which it did not pair, and complementary with its partner. 

 The sequence of nucleotides in the two strands will not be identical, but, 

 because of their complementarity, either one could generate both in two 

 cycles of replication. 



Less information is available about the native configuration of RNA 

 molecules, partly because, in contrast to DNA, they are not easily ex- 

 tracted from cells and purified without considerable structural damage. 

 A current view is that RNA is a single-stranded polymer, not helical like 



