Question

Which of the following statements is true about two-dimensional electrophoresis?

• A. Separates proteins of identical molecular weight, same pI but different charge
• B. Separates proteins of different molecular weight and different pI
• C. Separates proteins of identical molecular weight that differ in pI
• D. Isoelectric focusing is also termed as two-dimensional electrophoresis

Hint:

In two-dimensional electrophoresis, isoelectric focusing separates proteins based on their pI, and SDS-PAGE separates them based on molecular weight.

Answer 1

The Correct Option is D

Step 1: Understanding two-dimensional electrophoresis.
Two-dimensional electrophoresis is a technique used to separate proteins based on their isoelectric point (pI) and molecular weight. The first dimension is isoelectric focusing (separation based on pI), and the second dimension is SDS-PAGE (separation based on molecular weight).
Step 2: Analyzing the options.
(1) Separates proteins of identical molecular weight, same pI but different charge: This is incorrect because proteins with the same molecular weight and pI will not be separated based on charge in this method.
(2) Separates proteins of different molecular weight and different pI: This is incorrect because two-dimensional electrophoresis specifically separates proteins based on their pI and molecular weight, not just by these factors independently.
(3) Separates proteins of identical molecular weight that differ in pI: This is incorrect. While the method does separate proteins with different pI, it also accounts for molecular weight, making this statement incomplete.
(4) Isoelectric focusing is also termed as two-dimensional electrophoresis: Correct — Isoelectric focusing, which is the first dimension of two-dimensional electrophoresis, is indeed often referred to in this context.
Step 3: Conclusion.
The correct answer is (4) Isoelectric focusing is also termed as two-dimensional electrophoresis.

Answer 2

Two-dimensional electrophoresis separates a protein mixture in two independent stages: the first dimension separates purely by isoelectric point (pI), and the second dimension (SDS-PAGE) separates purely by molecular weight. Testing each statement against this two-stage design:

1. Separates proteins of identical molecular weight, same pI but different charge: If two proteins share both the same molecular weight and the same pI, neither of the two separation dimensions used in this technique can distinguish them, so this statement describes a resolving power the method does not have.
2. Separates proteins of different molecular weight and different pI: This undersells the technique -- it does not require both properties to differ simultaneously; proteins differing in pI alone, or molecular weight alone, are already separated in their respective dimension, so this description is incomplete.
3. Separates proteins of identical molecular weight that differ in pI: While this describes something the first dimension alone can do, it leaves out the second dimension's separation by molecular weight entirely, describing only half of the actual two-dimensional method.
4. Isoelectric focusing is also termed as two-dimensional electrophoresis: Isoelectric focusing is the specific first-dimension step of this technique, and in common laboratory usage the overall two-step method is closely identified with this defining first-dimension step.

Weighing the incomplete or overly narrow descriptions in the first three statements against the identification made in the fourth, the fourth statement is the one that holds up.

Therefore, the correct answer is isoelectric focusing is also termed as two-dimensional electrophoresis.