Q.75 E. coli ribonuclease contains 124 amino acids. The number of nucleotides present in the gene
encoding the protein is ____ .

E. coli ribonuclease with 124 amino acids requires a gene of 372 nucleotides, calculated from the genetic code where each amino acid is encoded by a triplet codon. This fundamental principle of molecular biology determines the coding sequence length.

Correct Answer

372 nucleotides. In the standard genetic code, each of the 124 amino acids in the protein is specified by one codon consisting of exactly 3 nucleotides in the mRNA transcribed from the gene. Thus, the minimal coding sequence (open reading frame) contains $124\times 3=372$ nucleotides, excluding untranslated regions, introns (absent in prokaryotes like E. coli), or stop codons in this calculation.​

Option Explanations

Typical multiple-choice options for this question test understanding of the triplet code versus common errors.

• 372 (Correct): Matches $124\times 3$, as each codon encodes one amino acid with no overlap or punctuation in prokaryotic protein-coding genes.​

• 123 or 124: Incorrect; confuses amino acids with nucleotides, ignoring the 3:1 ratio of nucleotides to amino acids.​

• 369: Wrong; subtracts one codon prematurely (e.g., assuming no stop codon counts toward amino acids, but stop codons don’t code for amino acids anyway—the 124 refers to the protein length).​

• 375: Incorrect; might add a stop codon (3 extra nucleotides), but the question specifies nucleotides “in the gene encoding the protein,” typically meaning the coding sequence for the amino acids only, not regulatory elements.​

Genetic Code Basics

Genes in E. coli, a prokaryote, lack introns, so the coding DNA directly translates to mRNA codons. One start codon (AUG, also codes for methionine) initiates, 122 internal codons specify the remaining amino acids, and a stop codon (UAA/UAG/UGA) terminates without adding an amino acid—yielding exactly 124 amino acids from 372 coding nucleotides. This principle applies universally across the degenerate 64-codon genetic code for 20 amino acids plus stops.​