Q.66 Most viral capsids have
(A) 08 faces (B) 12 faces (C) 16 faces (D) 20 faces
Viral capsids, especially icosahedral ones common in most viruses, feature 20 triangular faces for efficient nucleic acid enclosure, making (D) 20 faces the correct answer.
Correct Answer
Option (D) 20 faces is right. The icosahedron, the dominant capsid shape, has exactly 20 equilateral triangular faces, 12 vertices, and 30 edges, allowing stable assembly from repeating protein subunits (capsomeres).
Icosahedral Geometry Basics
Icosahedral capsids minimize surface area for volume, using 60T protein subunits where T=1 gives the simplest 60-subunit shell across 20 faces. Each face forms from structural units (often 3 subunits), enabling quasi-equivalence for self-assembly in viruses like adenoviruses and poliovirus.
Option Analysis
| Option | Faces | Explanation | Correct? |
|---|---|---|---|
| (A) 08 faces | 8 | Matches octahedron (6 vertices), rare in viruses; lacks icosahedral efficiency. | No |
| (B) 12 faces | 12 | Dodecahedron has 12 pentagonal faces, not triangular; not standard viral form. | No |
| (C) 16 faces | 16 | No common polyhedron; viruses avoid irregular counts for symmetry. | No |
| (D) 20 faces | 20 | Icosahedron standard: 20 triangles, 12 pentameric vertices; used by most non-enveloped viruses. | Yes |
