Virology technique: viruses can be purified on the basis of both size and density using which method?

Introduction / Context:
Purifying viruses from complex biological mixtures requires separating particles based on physical properties such as size and buoyant density. The technique chosen affects purity and yield. This question asks which method allows separation based on both size and density.

Given Data / Assumptions:

• Viruses vary in diameter, mass, and capsid composition, which affect sedimentation behavior.
• Common lab methods include differential centrifugation, gradient centrifugation (rate-zonal and isopycnic), and precipitation (for example, with polyethylene glycol).
• Goal: obtain a more homogeneous viral preparation while removing cellular debris and macromolecules.

Concept / Approach:
Gradient centrifugation uses a preformed gradient (sucrose, CsCl, iodixanol) so particles migrate at rates determined by size and density (rate-zonal) or band at the position where their buoyant density equals the gradient (isopycnic). This enables fine separation of virions from contaminants. Differential centrifugation uses sequential spins at increasing g-forces to sediment larger components first; it is cruder and primarily size-based. Precipitation concentrates particles nonspecifically and is not selective for precise size/density differences.

Step-by-Step Solution:

Verification / Alternative check:
Literature and protocols show viruses banding at distinct densities (for example, CsCl gradients) and fraction collection yielding purified virions.

Why Other Options Are Wrong:

• Differential centrifugation: Coarse fractionation; insufficient density resolution.
• Precipitation: Concentration method; lacks discriminative separation.
• None of these: Incorrect because gradient centrifugation fits perfectly.

Common Pitfalls:
Overloading gradients causing band overlap; using gradients incompatible with virus stability; misinterpreting rate-zonal vs isopycnic goals.

Final Answer:
gradient centrifugation