42 



Inside the Living Cell 



phosphate and sugar groups (Fig. 7). In addition each sugar molecule 

 has an organic *base' attached to it, of which there are commonly 

 four kinds, known as guanine (G), adenine (A), cytosine (C) and 

 thymine (T). It was at first thought that these four bases were present 

 in equal amounts, but careful analyses by Chargaff and others estab- 

 lished that this is not so, but they do occur in pairs, i.e. the amount 

 of adenine is always equal to the amount of thymine and the amount 

 of guanine is equal to the amount of cytosine. 



The reason for this was discovered by Crick and Watson in the 

 fact that DNA is made up of two threads in which the adenine of one 

 is always paired with thymine in the other and, similarly, guanine in 

 one thread is always paired with thymine in the other (see Fig. 8). 



y\ 



FIG. 7. Chemical composition of one thread of deoxyribonucleic acid 



They can do this because their chemical structures are exactly 

 complementary. Guanine can combine in a regular twin-threaded 

 structure with cytosine and only with cytosine, and the same is true 

 of adenine and thymine.^ 



In the structure proposed by Crick and Watson, the two threads 

 are wound round each other to form a spiral, but this is probably 

 not an important feature. 



What is much more important is that this structure provides a 

 basis for the reduplication of threads having bases in any particular 

 order. This is because, when they are combined in the way I have 

 described, each thread is the exact complement of the other, i.e. 

 wherever there is A in one thread, there is T in the other and 

 wherever there is G there is C in the other. 



A chromosome contains a large number of particles of dna and 

 it is tempting to identify each one with an independent gene. At the 

 ^ See Appendix (p. 165) for chemical formulae. 



