1) What is the minimum number of single nucleotide substitutions that would be necessary for each of the following amino acid replacements?

Question

a. trp -> lys
b. met -> his
c. tyr -> gly
d. ala -> asp
 
2) A mutation is isolated that can not be induced to revert. What type of molecular alteration might be responsible?
 
3) Which of the following amino acid  replacements would be expected with the highest frequency among mutations induced with 5BU: met -> leu, met -> lys, leu -> pro, pro -> thr,          thr -> arg

Can you help me with these? I don't get them at all. Thank You