Helical capsid example: which bacteriophage is a rod-shaped filament with capsomeres arranged helically rather than in stacked rings?

Introduction / Context:
Virus capsid symmetry is commonly icosahedral or helical. Filamentous bacteriophages are classic examples of helical symmetry, where identical capsid subunits wrap around the nucleic acid to form a rod-like particle. Recognizing exemplars helps connect morphology to life cycle and applications (for example, phage display).

Given Data / Assumptions:

• Rod-shaped viruses with helical symmetry have capsomeres arranged in a helix along the genome.
• T-even phages (T2, T4) are tailed, complex icosahedral heads, not simple helical rods.
• M13 is a filamentous (Ff) phage infecting E. coli bearing F pili.

Concept / Approach:
M13’s coat protein subunits polymerize around circular single-stranded DNA to form a flexible filament with helical symmetry. In contrast, T2/T4 possess icosahedral heads and contractile tails; their capsid proteins form facets, not continuous helices around the genome. Therefore, the correct helical rod example among the choices is M13.

Step-by-Step Solution:

Verification / Alternative check:
Structural images and cryo-EM reconstructions show M13 as a long filament with helical arrangement of coat proteins.

Why Other Options Are Wrong:

• Bacteriophage T2/T4: Complex morphology; not helical rods.
• MV-L2: Not the classic rod-shaped helical example compared with M13.

Common Pitfalls:
Confusing “helical symmetry” with “stacked ring” descriptions of some rod-like viruses; assuming all bacteriophages are tailed and icosahedral.

Final Answer:
Bateriophage M13