THE MUTABLE UNIT OF HEREDITY 59 



4. subdivisible, inasmuch as the mutable unit may be as small as 

 nucleotides whereas the gene need contain many nucleotides to possess 

 the specificity necessary for function. 



These are the properties of a macromolecule as revealed through the 

 study of mutation by biological methods, all the more remarkable be- 

 cause a given gene is represented by at most a few copies in each cell. 

 Mutations have a chemical mechanism, and the genetic material in its 

 alternative states controls the character of the cell by influencing its 

 biochemical behavior. These subjects will be dealt with in later 

 chapters. It is convenient first to continue our considerations of the 

 structure of genes and to turn to another biological method which 

 studies the recombinations that occur among them. 



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