Problem
The following sequence is that of the rst 45 codons from the human gene for preproinsulin. Using the standard genetic code determines what fraction of single mutations in the first and second positions of these 45 codons are synonymous.
ATG GCC CTG TGG ATG CGC CTC CTG CCC CTG CTG GCG CTG CTG GCC
CTC TGG GGA CCT GAC CCA GCC GCA GCC TTT GTG AAC CAA CAC CTG
TGC GGC TCA CAC CTG GTG GAA GCT CTC TAC CTA GTG TGC GGG GAA
It is known that among all the possible substitutions at the third position in the above codons, about 74.8% are synonymous and 25.2% are non-synonymous. Knowing this, and the percentage of synonymous substitutions you have calculated for the first and second positions above, answer the following question: At which codon position is natural selection likely to have the greatest effect and are nucleotides most likely to be conserved?