THE NATURE OF THE GENE 283 



account for points 3 and 4 of the foregoing enumeration. At the 

 present stage of our knowledge, we may assume, then, that a 

 gene, whatever it is, behaves in the way of an autocatalyst, 

 catalyzing all those reactions which we have studied in the 

 preceding chapters. 



To account for the facts of gene mutation and the details of 

 gene action, special assumptions have to be made, the more 

 important of which will now be analyzed. 



1. THE GENE AS AN ACTIVE MOLECULE OR GROUP OF 

 MOLECULES 



As the gene behaves as a unit in heredity, a unit located at a 

 definite locus in a chromosome, the simplest and most obvious 

 conception of its nature is the assumption that it is a separate 

 particle of matter, i.e., one or more molecules of a substance. If 

 this is the case, a number of possibilities are given for the explana- 

 tion of the process of mutation. The most important ones, at 

 the present stage of chemical knowledge, are as follows: 



1. A mutation is a change in the quantity of the gene, i.e., from 

 x to y molecules (Goldschmidt). 



2. A mutation is a complete change from one molecule into 

 quite another. 



3. A mutation is a partial change within the molecule, one 

 residue or side chain being replaced by another one (Guyer, 

 Correns, Demerec). 



4. A mutation is a change within the tridimensional arrange- 

 ment of a molecule, i.e., the formation of a stereoisomere 

 (Karczag). 



5. A mutation is a quantitative change, not by simple addition 

 or subtraction of individual molecules but by polymerisation, the 

 formation of chain molecules (Baur, etc.). We shall review these 

 possibilities after adding a few remarks concerning the number 

 of genes. 



Authors who assume that the gene is a molecule or a small 

 group of molecules are naturally interested in proving that the 

 size of the gene is of the order of magnitude of protein molecules. 

 Though calculations of this type may not seem so significant 

 now that it is known that chain molecules exist having a mag- 

 nitude of 800 to 1000A. length and more (see page 293), the facts 

 may be briefly mentioned. Estimates of the number of genes 



