MCAT Biology Question #6: Answer and Rationale

6. Sodium dodecyl sulfate (SDS) contains a 12-carbon tail attached to a sulfate group and is used in denaturing gel electrophoresis of proteins. Numerous SDS molecules will bind to the exposed hydrophobic regions of denatured proteins. The use of SDS in this experiment allows for the separation of proteins by: 

A) Charge

B) Molecular weight

C) Shape

D) Solubility

Answer Analysis

The correct answer is B.

Rationale: In this question, you must reason about the design and execution of research. You are also asked to recall knowledge about protein structure and function, as well as how gel electrophoresis works. In this technique, SDS detergent binds to proteins uniformly coating them in a negative charge and denaturing them, removing any three-dimensional shape. When a current is applied to the gel, the SDS-coated proteins migrate through the gel and towards the positively charged electrode. Proteins of lower molecular weight travel faster through the gel than those with greater molecular weight as smaller proteins have an easier time moving through the pores of the gel matrix. Based on this understanding, you can determine that, through this technique, proteins will be separated only by their molecular weight.

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