Question: The polypeptide chain in a protein has amino acids linked with each other in a specific sequence. This specific sequence of amino acids is called:

• (A) Primary structure of protein
• (B) Secondary structure of protein
• (C) Tertiary structure of protein
• (D) Quaternary structure of protein

Step 1 (Definition & Governing Principle): [0.5 Mark]

The structure of proteins is defined by the spatial arrangement and sequence of amino acids. The Primary Structure is defined as the specific sequence in which amino acids are linked together by peptide bonds ($-\text{CO}-\text{NH}-$ linkage) in a polypeptide chain.

Step 2 (Deductive Analysis): [0.25 Mark]

Evaluating the options based on NCERT definitions:

• Primary Structure: Refers strictly to the linear sequence of amino acids.
• Secondary Structure: Refers to the folding pattern (e.g., $\alpha$-helix or $\beta$-pleated sheet) due to hydrogen bonding.
• Tertiary Structure: Refers to the overall three-dimensional folding of the polypeptide chain.
• Quaternary Structure: Refers to the spatial arrangement of two or more polypeptide subunits.

Step 3 (Final Concluded Answer): [0.25 Mark]

Since the question specifies the "specific sequence of amino acids," the correct option is (A) Primary structure of protein.

💡 EL10 Concept Insight: Think of the primary structure like the specific order of letters in a word; if you change the order of the letters, the word (or in this case, the protein's function) changes completely!