Molecular Basis of Inheritance Practice Test

The key idea is how changes in the DNA sequence alter the reading frame and thus the sequence of amino acids in a protein. The reading frame is defined by grouping nucleotides into triplets (codons) that specify amino acids. If you insert or delete nucleotides in a number not divisible by three, every codon downstream shifts, so the amino acid sequence changes entirely from that point and a premature stop codon is often created. This frameshift typically produces a nonfunctional protein because most of the protein is misread and truncated. Conversely, insertions or deletions in multiples of three add or remove whole codons without changing the downstream reading frame, so the overall frame is preserved, though the protein gains or loses a few amino acids.

Single-nucleotide changes fall into three broad outcomes for the encoded amino acid: silent mutations don’t change the amino acid due to redundancy in the genetic code; missense mutations swap one amino acid for another; nonsense mutations convert a codon to a stop signal, truncating the protein. This combination of possibilities is why the statement that non-multiple-of-three indels cause a reading-frame shift (and that point mutations can be silent, missense, or nonsense) is correct.

Statements claiming the reading frame is unaffected by insertions/deletions or that all mutations inactivate proteins are not accurate, because frameshifts depend on the reading-frame impact of the indel, and many mutations have no effect or only a partial effect on protein function.