The genetic code is thought to have evolved to maximize genetic stability by minimizing the effect on protein function of most substitution mutations (single-base changes). We will use the six arginine codons to test this idea. Consider all of the substitutions that could affect all of the six arginine codons. Part A How many total mutations are possible?

Part B How many of these mutations are "silent," in the sense that the mutant codon is changed to another Arg codon

Part C How many of these mutations are conservative, in the sense that an Arg codon is changed to a functionally similar Lys codon?

The 4 codons that code for Alanine are :-

GCU ; GCC ; GCA ; GCG ;;

codon for Valine :-

GUU ; GUC ; GUA ; GUG

Part A. Total number of possible mutations are 36.

GCU

GCU --> GCC , GCA , GCG

GCU --> GUU , GAU , GGU

GCU --> UCU , CCU , ACU

GCC

GCC --> GCU , GCA , GCG

GCC --> GUC , GAC , GGC

GCC --> UCC , CCC , ACC

GCA

GCA --> GCU , GCC , GCG

GCA --> GUA , GAA , GGA

GCA --> UCA , CCA , ACA

GCG

GCG --> GCU , GCC , GCA

GCG --> GUG , GAG , GGG

GCG --> UCG , CCG , ACG

Part B. Silent mutations occur when the last base of the codon is substituted with other bases.

Thus, the Total number of silent mutations are 12.

Part C. Conservative mutations occur when the second base of the codon is substituted to a "U".

Thus, the Total number of conservative mutations is 4.