8 



Mutation as a Chemical Process 



We left the subject of mutation in Chapter 2 with the concept of the 

 mutable unit as a part of a molecule of nucleic acid which seemed 

 autonomously capable of oscillating with a long phase among a series 

 of chemically different states. This is not an exact picture, because the 

 gene is actually a much more dynamic entity, frequently engaged in its 

 own duplication and exercising its influence on whatever step it controls 

 in the biochemistry of the cell. As a consequence there are several ques- 

 tions we may ask. Does the gene mutate without replicating or is muta- 

 tion a copy-error occurring rarely during that process? Is mutation a 

 monomolecular event or one subject to the action of the cell's bio- 

 chemistry with which it is intimately involved? What can be said of the 

 mechanism of mutation? 



MUTATION AND GENE REPLICATION 



We will first consider whether gene replication is a necessary condi- 

 tion for mutation. Interesting studies have been performed by Novick 

 and Szilard with a device called the chemostat. This consists of a 

 chamber containing a suspension of bacteria (Figure 8.1). Nutrient solu- 

 tion drips in at a slow, constant rate, and at the same rate, the suspen- 

 sion leaks out through an overflow. If the solution entered the chamber 

 faster than the bacteria could divide to maintain their concentration, it 

 would wash them out. But when the bacteria can grow more rapidly 

 than the turnover rate, they exhaust the nutrient that is set as limiting 

 in the incoming solution and thereby maintain their concentration. For 

 every increment of bacteria newly formed, an equal lot is lost in the 



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