19^2 M. II. F. WILKIN S 



the sequence of atoms in one chain runs in opposite direction to that in the 

 other. As a result, one chain is identical with the other if turned upside- 

 down, and every nucleotide in the molecule has identical structure and envi- 

 ronment. The only irregularities arc in the base sequences. The sequence 

 along one chain can vary without restriction, but base-pairing requires that 

 adenine in one chain be linked to thymine in the other, and similarly guanine 

 to cytosine. The sequence in one chain is, therefore, determined by the se- 

 quence in the other, and is said to be complementary to it. 



The structure of the DN A molecule in the B configuration is shown in 

 Fig. 3. The bases are stacked on each other 3.4 A apart and their planes arc 

 almost perpendicular to the helix axis. The flat sides of the bases cannot bind 

 water molecules; as a result there is attraction between the bases when DNA 

 is in an aqueous medium. This hydrophobic bonding, together with the 

 base-pair hydrogen-bonding, stabilizes the structure. 



The Watson-Crick Hypothesis of DNA Replication, and Transfer of In- 

 formation from one Polynucleotide Chain to Another 



It is essential for genetic material to be able to make exact copies of itself; 

 otherwise growth would produce disorder, life could not originate, and 

 favourable forms would not be perpetuated by natural selection. Base- 

 pairing provides the means of self-replication (Watson and Crick 14 ). It also 

 appears to be the basis of information transfer during various stages in pro- 

 tein synthesis. 



Genetic information is written in a four-letter code in the sequence of the 

 four bases along a polynucleotide chain. This information may be transferred 

 from one polynucleotide chain to another. A polynucleotide chain acts as a 

 template on which nucleotides arc arranged to build a new chain. Provided 

 that the two-chain molecule so formed is exactly regular, base-pairing en- 

 sures that the sequence in the new chain is exactly complementary to that 

 in the parent chain. If the two chains then separate, the new chain can act as 

 a template, and a further chain is formed; this is identical with the original 

 chain. Most DNA molecules consist of two chains; clearly the copying 

 process can be used to replicate such a molecule. It can also be used to transfer 

 information from a DNA chain to an RNA chain (as is believed to be the 

 case in the formation of messenger RNA). 



Base-pairing also enables specific attachments to be made between part 



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