Wobble and degeneracy: Codons that specify the same amino acid usually differ in which nucleotide position?

Introduction / Context:
The genetic code is degenerate: most amino acids are encoded by more than one codon. The “wobble” hypothesis explains how a single tRNA can recognize multiple codons, typically differing at the third nucleotide.

Given Data / Assumptions:

• Coding is by triplets, positions numbered 1–3 from 5′ to 3′.
• tRNA anticodon pairing allows flexibility at the 3′ codon position.
• Question asks which position typically varies among synonymous codons.

Concept / Approach:
Because base pairing at the third codon position (5′ anticodon first position) is less stringent, many synonymous codons differ only in this “wobble” position, maintaining the same amino acid assignment while tolerating mutations.

Step-by-Step Solution:

Verification / Alternative check:
Codon tables across amino acids confirm third-position variability; exceptions (e.g., Ile AUA/AUC/AUU) still follow third-base differences.

Why Other Options Are Wrong:

• First or second base changes often alter the amino acid class or polarity and are less commonly synonymous.
• “None of these” contradicts widely accepted wobble rules.

Common Pitfalls:
Assuming all positions are equally variable without considering pairing rules; second position often correlates with amino acid hydrophobicity class.

Final Answer:
Third base (wobble position)