Questions: The Genetic Code: Reading Frame and Wobble Base Pairing

Question 1

A single nucleotide is inserted after position 15 in a 300-nucleotide coding sequence. What is the most likely effect on the protein?

• A. One amino acid is changed; all others remain the same, similar to a point mutation
• B. The first five codons are unaffected; only the altered codon and its immediate neighbors change
• C. Every codon from position 15 onward is scrambled, almost certainly destroying protein function
• D. The insertion is silent because wobble pairing compensates at the third position

Answer: (C)

A single nucleotide insertion shifts the reading frame at the insertion point. Every codon from that position to the end of the sequence is now read with a different grouping, producing a completely different amino acid sequence downstream. This is why frameshifts are typically far more damaging than point mutations — a point mutation affects at most one amino acid, and may even be silent due to codon degeneracy, while a frameshift scrambles the entire downstream sequence. Wobble pairing operates within the ribosome and does not compensate for frame errors.

Question 2

The human genome encodes about 45 tRNA species, yet there are 61 sense codons. How is this possible without mis-decoding?

• A. Many codons are never used in practice, so fewer tRNAs are sufficient
• B. A single tRNA can recognize multiple codons via wobble base pairing at the third codon position
• C. Ribosomes can skip codons that lack a matching tRNA, inserting a default amino acid
• D. Post-translational editing corrects amino acids inserted by imprecise tRNA recognition

Answer: (B)

Wobble base pairing allows the first position of the tRNA anticodon (which pairs with the third position of the mRNA codon) to form non-Watson-Crick pairs. For example, inosine (I) in the anticodon can pair with U, C, or A in the third codon position, allowing a single tRNA to recognize three synonymous codons. This is not imprecise — it is a precisely evolved mechanism that balances decoding efficiency with accuracy. It also explains why the third codon position is the 'wobble position' and why most synonymous codons differ only at position 3.

Question 3

Because the genetic code is degenerate, any point mutation at the third position of a codon usually produces a silent (synonymous) mutation.

• T. True
• F. False

Answer: False

While many third-position mutations are silent — especially in codon families where all four variants encode the same amino acid (like Ala: GCU, GCC, GCA, GCG) — this is not universal. In some codon families, the third position does distinguish amino acids. For example, CAA and CAG encode glutamine, but CAU and CAC encode histidine — a third-position A→U change in CA codons does change the amino acid. The degeneracy of the code means that third-position changes are often but not always silent.

Question 4

A deletion of exactly three consecutive nucleotides within a coding sequence is generally less damaging to protein function than a deletion of two consecutive nucleotides.

• T. True
• F. False

Answer: True

A 3-nucleotide deletion removes exactly one codon without shifting the reading frame for any downstream codons. The protein loses one amino acid at that position, and if the deletion is not in a critical region, function may be partially or fully preserved. A 2-nucleotide deletion shifts the reading frame, scrambling every codon from the deletion site to the end of the protein — an almost universally catastrophic outcome. Insertions and deletions are only tolerated when they occur in multiples of three.

Question 5

Explain why a frameshift mutation is typically far more damaging to protein function than a missense point mutation.

Model answer: A missense point mutation changes a single codon, altering one amino acid while leaving the rest of the protein intact. The effect depends on where the change is and how conservative the substitution is — many missense mutations are well tolerated. A frameshift (insertion or deletion not divisible by three) shifts the reading frame at the mutation site, changing every codon downstream. The entire amino acid sequence after the mutation point is scrambled, producing a completely different and almost certainly nonfunctional protein. Frameshifts also frequently introduce premature stop codons in the shifted frame, truncating the protein.

The key insight is that the reading frame is established at the start codon and maintained by the ribosome advancing exactly three nucleotides per codon. Any perturbation to the triplet grouping propagates downstream indefinitely. A point mutation is local (affects one codon); a frameshift is global (affects all downstream codons). This is why insertions/deletions of exactly three nucleotides are comparatively tolerable — they preserve the frame.