Genetic Code—Codon Size In the canonical genetic code used for translation, how many nucleic acid bases (nucleotides) specify one amino acid?

Introduction / Context:
A codon is the fundamental unit of the genetic code that directs insertion of a specific amino acid during translation. Determining the size of the codon was a crucial milestone in molecular biology, showing that a triplet of nucleotides is required to encode 20 amino acids with start/stop signals.

Given Data / Assumptions:

• There are four nucleotides (A, U/T, G, C).
• At least 20 amino acids must be encoded plus start/stop signals.
• The code is non-overlapping and nearly universal in cells.

Concept / Approach:
Calculate coding capacity: 1-base code allows 4 possibilities, 2-base code allows 4^2 = 16 (insufficient), and 3-base code allows 4^3 = 64, which can accommodate 20 amino acids plus redundancy (degeneracy) and stop codons. Empirical frameshift experiments and decoding studies confirm triplet codons in vivo.

Step-by-Step Solution:

Verification / Alternative check:
Cracking of the code (e.g., Nirenberg and Khorana experiments) directly demonstrated triplet codons directing specific amino acids.

Why Other Options Are Wrong:

• One or two bases cannot encode 20 amino acids.
• Four bases are unnecessary; nature uses triplets.
• None of the above: incorrect because "Three" is correct.

Common Pitfalls:
Confusing nucleotide triplets in mRNA (codons) with tRNA anticodons; they are complementary but both are triplet units.

Final Answer:
Three