Questions: Frameshift Mutations and Reading Frame Disruption

Question 1

A coding sequence contains the deletion of 4 nucleotides beginning at position 10. What is the most likely consequence for the encoded protein?

• A. One amino acid is removed from the protein but the rest of the sequence is unaffected
• B. All amino acids from position 4 onward are changed because the reading frame is shifted by 4
• C. All amino acids downstream of position 10 are changed because 4 is not divisible by 3, shifting the reading frame
• D. The mutation is silent because 4 nucleotides encode no complete codon

Answer: (C)

A deletion of 4 nucleotides — not divisible by 3 — shifts the reading frame. Every codon from the deletion site onward is composed of different nucleotides than before, encoding a completely different amino acid sequence. The ribosome reads in non-overlapping triplets from a fixed starting point; remove 4 nucleotides and all subsequent triplets shift by 4 mod 3 = 1 position, scrambling every downstream amino acid. Option A describes an in-frame deletion (divisible by 3). Option D is wrong — the ribosome does not skip; it continues reading in the now-shifted frame. Option B incorrectly states 'from position 4' — the effect begins at the site of the deletion.

Question 2

A frameshift mutation at position 50 of a 300-codon gene is compared to a missense mutation at the same position. Which best describes the difference in their effects on the protein?

• A. Both mutations affect only the single amino acid at position 50; the rest of the protein is unchanged
• B. The missense mutation changes all amino acids from position 50 onward; the frameshift only changes position 50
• C. The frameshift scrambles every amino acid from position 50 to the end of the protein; the missense mutation changes only the amino acid at position 50
• D. Both mutations are equally likely to introduce a premature stop codon

Answer: (C)

This is the key reason frameshifts are so catastrophic compared to point mutations. A missense mutation substitutes one nucleotide, changing one codon and thus one amino acid — the rest of the reading frame is intact. A frameshift shifts the entire downstream reading frame, meaning every codon from the mutation site onward encodes a different amino acid (or becomes a stop codon). The analogy: changing one letter in a sentence vs. removing a letter and re-spacing all subsequent words. Option B reverses the descriptions. Option D is wrong — missense mutations do not introduce stop codons by definition; frameshifts frequently encounter new stop codons in the shifted frame.

Question 3

A deletion of exactly 6 nucleotides from within a coding sequence will preserve the reading frame of all codons downstream of the deletion site.

• T. True
• F. False

Answer: True

Because the genetic code is read in non-overlapping triplets, removing a number of nucleotides that is a multiple of 3 removes complete codons without disrupting the alignment of downstream codons. A 6-nucleotide (2-codon) deletion removes two amino acids from the protein but leaves every subsequent codon intact. This is called an in-frame deletion, and its effect on protein function depends on which amino acids were deleted and whether they are structurally critical. In-frame deletions can be tolerated (the ΔF508 deletion in CFTR removes one amino acid but is not a frameshift) or devastating, but they are structurally distinct from frameshifts.

Question 4

A frameshift mutation affects primarily the amino acid encoded at the position of the insertion or deletion; downstream amino acids remain encoded by the original sequence.

• T. True
• F. False

Answer: False

This is the defining misconception about frameshifts. Because mRNA is read in consecutive, non-overlapping triplets from a fixed starting point, any shift in that starting point changes every codon that follows. Insert one nucleotide and every subsequent triplet is composed of different nucleotides — the reading frame is disrupted globally from the mutation site to the end of the mRNA. This is fundamentally different from a point mutation, which substitutes one nucleotide and thus affects only the codon containing it. The frame-wide disruption is why frameshifts cause loss-of-function in virtually all cases.

Question 5

Why does inserting or deleting a single nucleotide in a coding sequence alter every amino acid downstream of the mutation site, while inserting or deleting three nucleotides may not change any downstream amino acids?

Model answer: The genetic code is read in consecutive, non-overlapping triplets (codons) beginning from a fixed start point. Each codon's identity depends entirely on which three nucleotides are grouped together. Inserting or deleting one nucleotide shifts the grouping of all subsequent nucleotides by one position — every downstream codon is now composed of different nucleotides, encoding different amino acids (or a premature stop codon). The disruption propagates to the end of the mRNA because codons are read sequentially with no mechanism to re-establish the original frame. Inserting or deleting three nucleotides (one complete codon) adds or removes exactly one set of grouped nucleotides without shifting the register of any subsequent codon — downstream triplets are grouped the same way they were before.

The key concept is that codons are defined by their position within a reading frame, not by the nucleotide sequence alone. Shift the frame and every subsequent nucleotide is 'reassigned' to a different codon. This is why frameshift severity scales with how far downstream from the start the mutation occurs — mutations early in the gene corrupt more of the protein — and why in-frame indels (multiples of 3) are structurally distinct.