128 HOW ANIMALS DEVELOP 



are linked together end to end to form a chain. The 

 evidence about this comes largely from X-ray crystal- 

 lography. This shows that the type of nucleic acid 

 found in chromosomes (known as deoxyribose nucleic 

 acid, or DNA for short) has actually a slightly more 

 complicated structure than that just described. It con- 

 sists not of one sequence of nucleotides joined together 

 as a thread, but of two such threads wound round 

 each other in a spiral. However, these two intertwined 

 threads are closely related in their chemical composi- 

 tion; if, at a particular place in one thread, there is a 

 given constituent molecule or nucleotide, this imme- 

 diately determines the nature of the nucleotide at the 

 corresponding place in the other thread. Thus the 

 DNA, which we think to be the essential hereditary 

 substance, consists of a double thread built up from 

 pairs of nucleotides. 



This fits perfectly with the evidence from recombi- 

 nation which we discussed in the paragraph above. 

 The pairs of nucleotides which, according to the crys- 

 tallographers, are the units out of which the DNA 

 is built, are just about the same size as the smallest 

 stretches of chromosome which can be detected by 

 studying the recombination of hereditary factors. The 

 two stories dovetail together perfectly to give a coher- 

 ent picture. 



Now we have to go on to consider how this thread 

 of DNA produces a particular protein in the rest of 

 the cell. The first point is that proteins also, like 

 nucleic acids, are essentially thread-like molecules. 



