What is a reading frame shift and give an example of a consequence?

• A. A mutation that inserts/deletes nucleotides not divisible by 3, altering downstream codons and usually producing a nonfunctional protein
• B. A mutation that changes a single amino acid but preserves the reading frame
• C. A silent mutation with no amino acid change
• D. A mutation that creates a new start codon

Answer: (A)

The reading frame is how the genetic code is read in consecutive triplets during translation. A shift happens when nucleotides are inserted or deleted in a number not divisible by three, so every codon downstream is read in a new grouping. This changes which amino acids are added from that point on and almost always introduces a premature stop, producing a truncated, nonfunctional protein. For example, inserting a single nucleotide in a coding region alters all downstream codons, leading to a string of incorrect amino acids and likely early termination. If the insertion or deletion were in multiples of three, the frame would stay the same and you’d mainly gain or lose whole amino acids rather than corrupting the entire downstream sequence. Mutations that change a single amino acid without shifting the frame, or that don’t change the amino acid at all, don’t cause a reading frame shift, and creating a new start codon affects initiation rather than the reading frame.