Match the chemical (Column I) with its use (Column II).
Column I
P. Diethylpyrocarbonate
Q. Cesium chloride
R. Ethidium bromide
S. Ethylenediaminetetraacetic acid
Column II
1. Chelation of magnesium ion during DNA purification
2. Prevention of RNA degradation in aqueous environment
3. Separation of DNA by density gradient centrifugation
4. Staining of RNA in agarose gel
P-4; Q-1; R-3; S-2
P-4; Q-3; R-2; S-1
P-2; Q-1; R-4; S-3
P-2; Q-3; R-4; S-1
ANSWER
Correct match: P-2; Q-3; R-4; S-1, i.e., option (D).
Official answer: DSource: Official answer key, GATE 2024 BT (IISc Bengaluru)
P. Diethylpyrocarbonate (DEPC) covalently modifies histidine residues in RNases, inactivating them; DEPC-treated water is the standard reagent used to keep aqueous solutions free of RNase activity, i.e., it prevents RNA degradation in aqueous environments → P-2.
Q. Cesium chloride (CsCl), when subjected to ultracentrifugation, forms a continuous density gradient; nucleic acids (DNA, often visualized with an intercalating dye) band at their characteristic buoyant density in this gradient — the classical CsCl density-gradient centrifugation method for separating/purifying DNA → Q-3.
R. Ethidium bromide is a planar aromatic dye that intercalates between stacked bases of nucleic acids and fluoresces under UV light; it is the standard stain used to visualize nucleic acids (here, RNA) resolved on an agarose gel → R-4.
S. Ethylenediaminetetraacetic acid (EDTA) is a chelating agent that sequesters divalent cations, particularly Mg²⁺; because many nucleases (DNases) require Mg²⁺ as a catalytic cofactor, EDTA inhibits nuclease activity and thereby protects DNA during purification → S-1.
Combining the matches: P-2; Q-3; R-4; S-1.
This combination corresponds exactly to option (D).
KEY IDEA
Match each reagent to its standard molecular-biology use: DEPC inactivates RNases (protects RNA in solution), CsCl forms a density gradient for DNA separation by ultracentrifugation, ethidium bromide intercalates into nucleic acids and is used for staining in gels, and EDTA chelates Mg²⁺ to inhibit nuclease activity during DNA purification.
WATCH OUT FOR
Confusing ethidium bromide's role (a DNA/nucleic-acid intercalating stain used generically, including with agarose gels for RNA when specified) with EDTA's chelation function, or forgetting that CsCl requires ultracentrifugation to form the density gradient that separates DNA by buoyant density — leading to swapped R/S or Q/S assignments.
From the GATE 2024 BT paper. Every question of this paper · Download the full paper · All 8 PCR & sequencing questions, with notes
