If a bacterial protein contains 30 amino acids, how many nucleotide bases are needed to code for it?

• A. 30
• B. 90
• C. 60
• D. 120

Answer: (B)

Proteins are built from amino acids encoded by codons, with each codon made of three nucleotides. So to specify 30 amino acids, you need 30 codons, which is 30 × 3 = 90 nucleotide bases in the coding sequence. The start codon initiates translation and the stop codon terminates it, but the stop codon doesn’t code for an amino acid; it would only add bases if you counted it separately. Here, the bases required to code the amino acid sequence are 90.