In SDS-PAGE (Sodium Dobecyl Sulfate-polyacrelamide Gel Electrophoresis) proteins are separated on basis of
## **Core Concept**
SDS-PAGE (Sodium Dodecyl Sulfate-polyacrylamide Gel Electrophoresis) is a technique used in biochemistry to separate proteins based on their size (molecular weight). The method involves denaturing proteins with SDS, a detergent that disrupts non-covalent bonds, and then separating them according to their size as they migrate through a gel matrix under an electric field.
## **Why the Correct Answer is Right**
The correct answer, **B. Molecular size (or molecular weight)**, is right because in SDS-PAGE, SDS binds to proteins in a ratio of approximately 1.4 grams of SDS per gram of protein, giving them a uniform negative charge-to-mass ratio. As a result, when an electric field is applied, the proteins migrate through the gel matrix at rates that are inversely proportional to the logarithm of their molecular weights, effectively separating them based on size.
## **Why Each Wrong Option is Incorrect**
- **Option A:** Charge is incorrect because, although proteins have different charges, the SDS used in SDS-PAGE masks these charges by binding to the proteins and giving them a uniform negative charge. This uniform charge-to-mass ratio ensures that proteins are separated based on size rather than charge.
- **Option C:** Shape is incorrect because, while the shape of a protein can influence its migration in some types of gel electrophoresis, SDS-PAGE effectively denatures proteins into a rod-like shape due to the action of SDS. This makes shape a negligible factor in the separation process.
- **Option D:** Density is incorrect because, although density can affect the migration of particles in some types of separations, SDS-PAGE separates proteins primarily based on their size as they move through the gel under the influence of an electric field, not their density.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that SDS-PAGE is a powerful tool for estimating the molecular weight of proteins and assessing protein purity. It's widely used in molecular biology and biochemistry for these purposes.
## **Correct Answer:** B. Molecular size (or molecular weight).