Question

Which of the following is a structurally independent three-dimensional unit associated with a particular functional role?

• A. Domain
• B. Motif
• C. Strand
• D. Helix

Hint:

• Helix/Strand → Secondary structure
• Motif → Pattern
• Domain → Functional 3D unit

Answer 1

The Correct Option is A

Concept: Proteins have hierarchical structural organization:
• Secondary structure: Helix, strand
• Motif: Combination of secondary structures
• Domain: Independently folded, functional 3D unit

Step 1: Analyzing each option.

• Domain: A stable, independently folding 3D unit with a specific function → Correct.
• Motif: A recurring structural pattern but not independently stable → Incorrect.
• Strand: A secondary structure element (part of $\beta$-sheet) → Incorrect.
• Helix: A secondary structure element (e.g., $\alpha$-helix) → Incorrect.

Step 2: Selecting the correct answer.
Only domain represents an independently folded 3D unit with a specific function.