How many different tripeptides can be formed from three different amino acids if each tripeptide contains each of the three amino acids once?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 6
A tripeptide is three amino acids joined by two peptide bonds. Each peptide bond joins the –COOH group of one amino acid to the –NH2 group of the next, so a chain has a free amino end and a free carboxyl end, and A–B–C is a different tripeptide from C–B–A. For three different amino acids A, B and C, each used once:
A first: A–B–C, A–C–B → 2 tripeptides.
B first: B–A–C, B–C–A → 2 tripeptides.
C first: C–A–B, C–B–A → 2 tripeptides.
Total: 3 × 2 × 1 = 6 tripeptides.
NCERT (2026-27 edition):
10.2.3 Structure of Proteins – “If a third amino acid combines to a dipeptide, the product is called a tripeptide. A tripeptide contains three amino acids linked by two peptide linkages.”
“(i) Primary structure of proteins: … Each polypeptide in a protein has amino acids linked with each other in a specific sequence … Any change in this primary structure i.e., the sequence of amino acids creates a different protein.”