Q.40

Three samples of antibodies were electrophoresed under denaturing and reducing conditions
on a 15% acrylamide gel, followed by staining with Coomassie blue dye.
Samples 1, 2 and 3 showed two, three and four stainable bands respectively.
Which one of the following conclusions can be made from these observations?

Options:

(A) Sample 1 is IgG, 2 is IgA and 3 is IgM

(B) Sample 1 is IgA, 2 is IgM and 3 is IgG

(C) Sample 1 is IgG, 2 is IgM and 3 is IgA

(D) Sample 1 is IgA, 2 is IgG and 3 is IgM

Introduction

In immunology and biochemistry, SDS-PAGE under denaturing and reducing conditions is commonly used to analyze the subunit composition of antibodies. Different classes of immunoglobulins (IgG, IgA, IgM) have different numbers of heavy chains, light chains, and additional linking components, which result in distinct band patterns on electrophoresis.
This question tests your understanding of antibody structure and how it reflects on gel electrophoresis results.

Key Concept: Antibody Structure Under Reducing Conditions

Under reducing conditions:

• Disulfide bonds break

• Antibodies separate into individual polypeptide chains

• Each distinct polypeptide appears as a separate band

Band Patterns of Common Immunoglobulins

IgG

• Structure: 2 heavy chains + 2 light chains

• Heavy chains are identical, light chains are identical

• Total distinct chains = 2

• Bands observed = 2

IgA (Secretory form)

• Structure includes:

  • Heavy chain

  • Light chain

  • J chain

• Total distinct chains = 3

• Bands observed = 3

IgM

• Pentameric structure

• Contains:

  • Heavy chains

  • Light chains

  • J chain

  • Additional structural complexity

• Total distinct chains = 4

• Bands observed = 4

Matching Samples with Antibodies

✅ Correct Answer

Option (A): Sample 1 is IgG, Sample 2 is IgA, Sample 3 is IgM

Explanation of All Options

Option (A)

✅ Correct
Band numbers exactly match the known structural subunits of IgG, IgA, and IgM.

Option (B)

❌ Incorrect
IgA does not produce only two bands; IgG does.

Option (C)

❌ Incorrect
IgM produces the highest number of bands, not IgA.

Option (D)

❌ Incorrect
IgA cannot give only two bands, and IgG cannot give three.

Quick Reference Table

Final Conclusion

SDS-PAGE under reducing conditions clearly differentiates antibodies based on their polypeptide composition. The observed band pattern confirms that:

Sample 1 = IgG,   Sample 2 = IgA,   Sample 3 = IgM​