4.
In a new species of bacteria, tRNA anticodons are recognized by two nucleotide bases
instead of three. What would be the number of amino acids and the number of peptide
bonds, respectively, of a polypeptide encoded by a 120 nucleotide long coding DNA
sequence which includes a two-base stop codon?
a. 58, 58
b. 59, 58
c. 60, 59
d. 61, 60
Two-Base Codon Genetic Code in Bacteria: Amino Acids and Peptide Bonds Calculation
In this CSIR NET Life Sciences question, a novel bacterial species uses two-nucleotide codons instead of three, with tRNA anticodons recognizing two bases and a two-base stop codon terminating translation for a 120-nucleotide coding DNA sequence.
Codon Length Logic
Standard genetic codes use triplet codons (3 nucleotides per amino acid), yielding 64 codons (4³) for 20 amino acids plus 3 stops. Here, two-base codons provide 16 possible combinations (4²), sufficient for amino acids plus one two-base stop codon. The 120-nucleotide coding sequence (CDS) thus encodes codons read in pairs until the stop.
Calculation Breakdown
Total codons = 120 nucleotides ÷ 2 nt/codon = 60 codons. Subtract 1 stop codon, leaving 59 codons for amino acids. A polypeptide with 59 amino acids forms 58 peptide bonds (n-1 bonds link n residues).
Option Analysis
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a. 58, 58: Assumes 59 codons total (118 nt coding + 2 nt stop), ignoring full 120 nt usage.
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b. 59, 58: Correct—59 amino acid codons + 1 stop codon = 60 × 2 = 120 nt; 58 bonds between 59 residues.
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c. 60, 59: Treats all 60 as amino acids, omitting stop codon role.
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d. 61, 60: Implies 122 nt, exceeding 120 nt limit.
