When a base substitution mutation occurs, one nucleotide in a replicating dna sequence is substituted for another, which results in the production of a mutant strand of dna. the result of the mutation depends on how the substituted nucleotide base alters the string of amino acids coded by the mutant dna. the three types of base substitution mutations are nonsense mutations, missense mutations, and silent mutations. each type is defined by how it affects protein synthesis?