Questions: Wobble Base Pairing and Codon Flexibility

Question 1

A tRNA has the anticodon 3'-CGI-5' (where I is inosine at the wobble position). During translation, which mRNA codons can this tRNA recognize?

• A. Only GCG — inosine pairs exclusively with cytosine
• B. GCU, GCC, and GCA — inosine at the wobble position pairs with U, C, or A in the third codon position
• C. GCU, GCC, GCA, and GCG — inosine pairs with all four nucleotides
• D. Only GCU — inosine strictly replaces uracil

Answer: (B)

The anticodon 3'-CGI-5' pairs with codons 5'-GCN-3' where N is at the wobble (third) position. Inosine can pair with U, C, or A — but not G. So this tRNA reads GCU, GCC, and GCA (three of the four alanine codons). GCG requires a separate tRNA. This illustrates how a single tRNA with inosine can cover multiple synonymous codons, reducing the total tRNA count needed.

Question 2

A mutation changes a codon from GCU to GCC in a protein-coding gene. Both codons encode alanine. What is the most likely effect on the protein?

• A. The protein is non-functional because the codon sequence changed
• B. No change in protein sequence — wobble base pairing allows the same tRNA (with inosine at the wobble position) to read both codons, inserting alanine in both cases
• C. A different amino acid is inserted because GCC is a different codon from GCU
• D. Translation terminates at the mutated codon

Answer: (B)

GCU → GCC is a synonymous (silent) mutation at the third codon position — the only position where wobble pairing applies. A tRNA with inosine at its wobble position reads both GCU and GCC, inserting alanine each time. The protein is identical. This illustrates the buffering role of wobble pairing: third-position point mutations are often silent precisely because the third position is the 'wobbly' one.

Question 3

Wobble base pairing occurs at most three positions of the codon-anticodon interaction, which is why the genetic code is degenerate at most three positions.

• T. True
• F. False

Answer: False

Wobble pairing occurs only at the third codon position (paired with the first anticodon position). The first two codon positions require strict Watson-Crick base pairing (A-U, G-C), which is why they carry most of the coding specificity. If wobble occurred at all three positions, a single tRNA could read many unrelated codons, catastrophically reducing translation fidelity. Degeneracy is concentrated at the third position precisely because that is the only position where non-standard pairing is tolerated.

Question 4

The clustering of synonymous codons — where codons for the same amino acid typically differ only at the third base — is directly explained by wobble base pairing at that position.

• T. True
• F. False

Answer: True

Wobble pairing makes the third codon position tolerant of variation: a single tRNA can read multiple codons differing there. The genetic code is structured so that degeneracy concentrates at the third position precisely because this is the 'wobbly' position where variation least disrupts tRNA recognition. This also means third-position point mutations are often silent — they produce synonymous codons for the same amino acid — buffering the proteome against random mutations.

Question 5

Why does the cell need far fewer than 61 tRNA species to decode all 61 sense codons, and what determines the minimum number actually required?

Model answer: Wobble base pairing allows one tRNA to recognize multiple codons differing at the third position (e.g., a tRNA with inosine can pair with codons ending in U, C, or A). Because each tRNA can cover 2–3 synonymous codons, fewer distinct anticodons are needed. The minimum is set by the wobble pairing rules: together, all tRNA anticodons must cover all 61 sense codons, but each can cover more than one, yielding ~31–45 tRNAs in practice.

The exact tRNA count varies by organism because different wobble pairs are used, and some organisms rely more on anticodon base modifications (like inosine generated by ADAT enzymes). The key insight is that 61 tRNAs would be required only if every codon needed its own perfectly complementary anticodon — wobble pairing breaks that one-to-one requirement at the third position, dramatically reducing the tRNA repertoire the cell must maintain.