SDS–PAGE characteristics — Identify the set of statements that correctly describe protein behavior in SDS–PAGE
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AProteins are denatured by SDS
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BProteins acquire an approximately constant charge-to-mass ratio
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CSmaller polypeptides migrate more rapidly through the gel matrix
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DAll of the above
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ENone of the above (SDS–PAGE preserves native complexes)
Answer
Correct Answer: All of the above
Explanation
Introduction:This question checks recognition of three hallmark features of SDS–PAGE: denaturation, charge normalization, and size-dependent mobility. Understanding these together explains why SDS–PAGE estimates apparent molecular masses reliably.
Given Data / Assumptions:
- SDS is an anionic detergent that unfolds proteins.
- SDS binding is roughly proportional to length, producing near-constant charge-to-mass ratios.
- Polyacrylamide gels sieve molecules by size under an electric field.
Concept / Approach:Combine the chemical action of SDS with the physical sieving of the gel. Denatured chains with similar charge density migrate according to length, so smaller chains move faster and farther during a set run time.
Step-by-Step Solution:
1) Confirm denaturation effect (true) → supports removal of shape effects.2) Confirm constant charge density (true) → eliminates native charge differences.3) Confirm size dependence (true) → explains separation principle.Verification / Alternative check:Standard marker ladders yield linear relationships between distance migrated and log(MW), empirically validating the trio of statements.
Why Other Options Are Wrong:
e) SDS–PAGE does not preserve native assemblies; that is a property of native PAGE or BN-PAGE.Common Pitfalls:Assuming reducing agents are always required; some analyses run non-reducing SDS–PAGE when disulfide status is of interest.
Final Answer:All of the above.