Phage morphology: Which bacteriophages are described as having binal (dual) symmetry, with an icosahedral head and a helical tail?
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AT2
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BAll of these
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CT6
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DT4
Answer
Correct Answer: All of these
Explanation
Introduction / Context:Bacteriophages (phages) exhibit diverse morphologies. A common architecture among tailed phages is “binal symmetry,” combining two structural symmetries: an icosahedral capsid (head) attached to a helical tail apparatus for adsorption and genome injection.
Given Data / Assumptions:
- The question focuses on classic Escherichia coli T-even/T-series phages.
- “Binal” (sometimes misspelled as “banal”) refers to dual symmetry.
- We consider T2, T4, and T6 morphologies.
Concept / Approach:Tailed dsDNA phages, including T2, T4, and T6, share the canonical structure: icosahedral head containing genomic DNA and a contractile or non-contractile tail with baseplate and tail fibers. This dual-symmetry design underlies their infection mechanism: tail fibers bind receptors, the tail sheath may contract, and DNA is injected into the bacterial cytoplasm.
Step-by-Step Solution:
Identify structural classes: head–tail phages have binal symmetry.Recall that T2, T4, T6 are tailed coliphages with similar architecture.Therefore, each listed T-series phage exhibits binal symmetry.Choose the inclusive option “All of these.”Verification / Alternative check:Electron micrographs and standard microbiology texts depict T-even phages as icosahedral-headed with distinctive tails and fibers.
Why Other Options Are Wrong:
- Each single-phage option is only partially correct; all three share the morphology.
Common Pitfalls:Confusing “binal” with “bilateral”; binal specifically refers to combined icosahedral and helical symmetry in phages.
Final Answer:All of these