38. In a mixture of the five proteins listed below, which should elute second in size-exclusion (gel-filtration) chromatography?
A. Cytochrome c (Mw = 13,000)
B. Immunoglobulin G (Mw = 145,000)
C. Ribonuclease A (Mw = 13,700)
D. RNA polymerase (Mw = 450,000)
Size-Exclusion Chromatography Protein Elution Order
In size-exclusion chromatography, proteins elute by decreasing molecular weight since larger ones are excluded from column pores and exit first. Among these five proteins, immunoglobulin G (Mw = 145,000) elutes second after RNA polymerase (450,000).
SEC Principles
Size-exclusion (gel-filtration) chromatography separates proteins by hydrodynamic volume. Porous beads allow small proteins to enter pores (longer path, later elution), while large ones travel the void volume (shorter path, earlier elution). Elution order is strictly largest to smallest MW.
Protein MW Ranking
Sorted by decreasing MW for elution sequence:
| Protein | Mw | Elution Position |
|---|---|---|
| RNA polymerase | 450,000 | First |
| Immunoglobulin G | 145,000 | Second |
| Ribonuclease A | 13,700 | Third |
| Cytochrome c | 13,000 | Fourth |
Option Analysis
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A. Cytochrome c (13,000 Da): Smallest; enters all pores, elutes last (fourth). Too small for second place.
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B. Immunoglobulin G (145,000 Da): Second-largest; excluded from some pores, elutes promptly after largest. Correct choice.
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C. Ribonuclease A (13,700 Da): Nearly identical to cytochrome c; elutes third due to slight size edge, but still late.
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D. RNA polymerase (450,000 Da): Largest complex; fully excluded, elutes first (void volume).
Why IgG is Second
RNA polymerase (450 kDa) leads as the biggest. IgG (145 kDa) follows since no intermediates exist—next are ~13 kDa proteins bunched at the end. SEC calibration curves confirm this inverse elution vs. log(MW). Ideal for GATE prep on chromatography basics.
1 Comment
Vanshika Sharma
February 3, 2026Immunoglobin will elute secondly from column