THE FINAL ADJUSTMENTS 127 



recombinations occurring in sections of chromosome 

 which were so short that a breakage and rejoining 

 would only occur in that place with a frequency of, 

 say, once in a million. If we can find that there are two 

 hereditary factors with recognisably different effects, 

 lying so near together that recombination between 

 them happens only once in a million times, we can 

 from this deduce something about the minimum size 

 of the units out of which the chromosome is built. It 

 turned out, in fact, that the size of the units from 

 which the hereditary material is made is about that of 

 a moderate-sized organic chemical molecule. 



The next step in the argument is to determine what 

 sort of chemical compounds are present in the chromo- 

 somes. In fact there are two: proteins and another class 

 of compounds known as nucleic acids. Which of these 

 two is the more important, or are they perhaps both 

 equally important? Since proteins are, as we have seen, 

 essential for so much of the lif^ of the cell, it might be 

 thought at first that they would play the essential 

 role in heredity also, but the evidence is actually on 

 the other side. In experiments with bacteria it has 

 been possible to transfer a hereditary characteristic 

 from one strain to another by using a pure preparation 

 of nucleic acids, but it has never been possible to do 

 this by using pure proteins. It seems then that it is the 

 nucleic acid which is the important constituent of the 

 chromosome in determining its hereditary effect. 



Nucleic acids are made up of a long sequence of rela- 

 tively small molecules (known as nucleotides) which 



