THE UNIQUENESS OF THE INDIVIDUAL 



in modern jargon, acts as the carrier of genetic information. 

 But what, or of what nature, is the code? Nucleic acids are 

 essentially polymers, or repetitive manifolds, of simpler units 

 known as nucleotides, and each nucleotide is a compound 

 between phosphoric acid, a sugar molecule with five carbon 

 atoms (unlike glucose, which has six, and ordinary cane sugar, 

 twelve), and an organic base which belongs to one or other of 

 two families, the purines and pyrimidines. In desoxyribonucleic 

 acid, the one we are concerned with, there appear to be four 

 possible bases, if we count two closely related bases as one: the 

 two purines are adenine and guanine, the two p}Timidines 

 thymine and cytosine. The constituent nucleotides are so 

 arranged that the sugar group of one attaches to the phosphate 

 group of its neighbour; the backbone of the polymer is there- 

 fore an alternating sequence of phosphate and sugar groups. 

 But what is all important is their arrangement. According to 

 the analyses of Watson and Crick,^ helped by the X-ray 

 diffraction studies carried out at King''s College, the molecule 

 of desoxyribonucleic acid is a pair of threads, each having the 

 polymeric structure just described. The threads lie equidistant 

 from each other, and are twisted to form a double spiral; and 

 (still following Watson and Crick) matters are so disposed that 

 the adenine base of the one thread is linked to a thymine base 

 on the other, and the guanine of one to the cytosine of the 

 other. The two threads are therefore of complementary struc- 

 ture: given the sequence of bases on one, the sequence on the 

 other is automatically fixed. This arrangement makes it 

 possible to envisage how the molecule is reproduced, for if each 

 of the two constituent threads builds up a complementary 

 thread upon itself, the four may now split lengthwise along the 

 line of union between the original two, so giving two double 

 threads, identical with each other and with the double thread 

 from which they first arose. But this does not solve the problem 

 1 J. D. Watson and F. H. C. Crick, Nature, 171, p. 964, 1953. 



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