Molecular Genetics Practice Exam

Genetic code degeneracy means different codons can code for the same amino acid. A silent mutation is a DNA base change that produces a codon still encoding the same amino acid, so the resulting protein sequence stays the same. For example, a codon change from GGU to GGC both specify glycine, so the amino acid sequence is unchanged. While the protein composition remains identical, silent mutations can sometimes affect gene expression details like mRNA structure or splicing in certain contexts, but the amino acid sequence remains unaffected. In contrast, a missense mutation changes a codon to one that encodes a different amino acid, altering the protein sequence. A nonsense mutation turns a codon into a stop signal, truncating the protein. A frameshift mutation inserts or deletes nucleotides not in multiples of three, shifting the entire downstream reading frame and usually producing a markedly different, often nonfunctional protein.