Chromosomes and Genes 101 



These two equations are found to be identical if the number of hits 

 is one. 



These deliberations led to the construction of a model. It is 

 supposed that the gene is a very stable molecule, a combination of 

 atoms with definite positions and electronic conditions. Such a system 

 might be changed in diflFerent ways, the most important of which 

 is the dissipation of energy of excitation. This leads to an ionization 

 in the neighborhood, a rearrangement of the atomic complex by a 

 single elementary process. A comparison of the energy requirements 

 with certain facts of mutation shows a qualitative agreement. 



The latter comparison is based upon the facts of spontaneous 

 mutation. Changes within a molecule may be produced not only by 

 external energy, like radiations, but also by the statistical variation 

 of the temperature oscillations between the atoms. The speed of 

 such changes is known to be dependent upon temperature and 

 energy of activation for the start of the reaction. The temperature 

 relations are expressed in the rule of Arrhenius-vant'Hoff, which 

 establishes a relation between energy of activation, stability of the 

 molecule, and the temperature coefficient. The last is in many chemi- 

 cal reactions near 2-3 for a 10° difference in temperature. From this 

 it follows that mutability, if based upon such a process, must have 

 a temperature coefficient. This is the case, as Muller first noticed 

 (1928). The coefficient was found to be in the neighborhood of 5; 

 in a more recent report by Timofeeff and Zimmer (1947) it was 

 6.5. From the actual data, the energy of activation (needed to over- 

 come the stabilizing forces in the molecule) can be calculated for 

 the normal mutation rate as well as for the rate increased according 

 to the temperature coefficient. The two values were found to be 

 the same (under the assumption of 500 genes with 20 alleles in the 

 X-chromosome ) , approximately 1.4 ev. Delbriick now calculated the 

 expected temperature coefficient of the mutation rate as a measure of 

 the rate of such molecular changes, to be accomplished when the 

 amount of energy of activation was eventually reached. A tempera- 

 ture coefficient of 5 was expected, approximately the value that was 

 actually found. 



From these facts the conclusion was drawn that mutation consists 

 in a change of equilibrium between the atoms, produced by temper- 

 ature oscillations or an outside energy source. The primary process of 

 absorption of a quantum might lead to very different secondary 

 processes, such as simple steric rearrangements or dissociation of 



