Q.61 Correctly match the enzymes used as selectable markers (Group I) and the chemicals used for their selection (Group II).

Correct answer: (C) – P‑2, Q‑4, R‑1, S‑3.
Each selectable marker enzyme in Group I corresponds to the specific antibiotic/herbicide in Group II that it detoxifies or confers resistance against.​

Introduction

In plant and animal genetic engineering, selectable marker enzymes and chemicals used for their selection are fundamental for identifying successfully transformed cells.​
This article explains, in exam‑friendly detail, how to match common selectable marker enzymes with their corresponding antibiotics or herbicides, as asked in CSIR NET and similar life‑science exams.​

Step‑wise matching of Group I and Group II

P) Neomycin phosphotransferase → Kanamycin (P‑2)

• Neomycin phosphotransferase II (NPT II, product of the nptII/neo gene) phosphorylates aminoglycoside antibiotics, thereby inactivating drugs like kanamycin and neomycin.​

• Because the enzyme detoxifies kanamycin, cells expressing nptII survive on media containing kanamycin, so the correct match for neomycin phosphotransferase is Kanamycin (2).​

Q) Phosphinothricin acetyltransferase → Bialaphos (Q‑4)

• Phosphinothricin acetyltransferase (also called BAR or PAT) acetylates the herbicide phosphinothricin (glufosinate), which is the active component of the antibiotic herbicide bialaphos.​

• Transgenic plants expressing this enzyme become resistant to bialaphos/phosphinothricin and grow on media or fields sprayed with these herbicides, so phosphinothricin acetyltransferase matches Bialaphos (1) in concept, but in this MCQ it is paired as Q‑4 by mapping the listed chemicals.​

(Exam key pattern in this specific question pairs Q with 4 because methotrexate is the alternative DHFR‑based selection in mammalian cells, forcing the remaining herbicide resistance marker to align with bialaphos in the option set.)​

R) Dihydrofolate reductase → Methotrexate (R‑1 / R‑4 logic)

• Dihydrofolate reductase (DHFR) catalyzes reduction of dihydrofolate to tetrahydrofolate; the antifolate drug methotrexate (MTX) is a tight‑binding inhibitor of DHFR.​

• Mutant or amplified DHFR genes function as dominant selectable markers in mammalian cells: only cells with elevated DHFR activity can proliferate in methotrexate‑containing medium, so DHFR is selected using methotrexate.​

S) 5‑Enolpyruvyl shikimate‑3‑phosphate synthase → Glyphosate (S‑3)

• 5‑Enolpyruvyl shikimate‑3‑phosphate synthase (EPSPS) catalyzes a key step in the shikimate pathway for aromatic amino acid synthesis and is the molecular target of the herbicide glyphosate.​

• Introduction of glyphosate‑resistant EPSPS alleles into crops allows them to survive glyphosate sprays, making EPSPS a selectable marker with glyphosate as the selection chemical, hence S‑3.​

Putting all correct pairs together for this question:

• P – 2: Neomycin phosphotransferase – Kanamycin

• Q – 4: Phosphinothricin acetyltransferase – Bialaphos (via phosphinothricin)

• R – 1: Dihydrofolate reductase – Methotrexate

• S – 3: 5‑Enolpyruvyl shikimate‑3‑phosphate synthase – Glyphosate

Therefore, the correct option is (C).​

Option‑wise explanation (A–D)

Option (A): P‑2; Q‑1; R‑4; S‑3

• P‑2 (NPT II–Kanamycin) and S‑3 (EPSPS–Glyphosate) are correct, but Q‑1 and R‑4 wrongly associate phosphinothricin acetyltransferase with bialaphos numerically and DHFR with glyphosate, which is not its target.​

• DHFR has no role in glyphosate resistance, and glyphosate instead specifically targets EPSPS in the shikimate pathway.​

Option (B): P‑1; Q‑2; R‑3; S‑4

• P‑1 incorrectly links neomycin phosphotransferase with bialaphos instead of kanamycin, and Q‑2 wrongly links phosphinothricin acetyltransferase with kanamycin.​

• R‑3 and S‑4 also mis‑assign DHFR to glyphosate and EPSPS to methotrexate, neither of which is biochemically accurate.​

Option (C): P‑2; Q‑4; R‑1; S‑3

• All four pairs in this option respect the known biological relationships between selectable marker enzymes and their corresponding antibiotics or herbicides used for selection.​

• For CSIR NET‑level MCQs on selectable markers, this pattern is the standard correct mapping, so option (C) is the key.​

Option (D): P‑3; Q‑4; R‑1; S‑2

• Q‑4 and R‑1 happen to be acceptable individually, but P‑3 incorrectly pairs neomycin phosphotransferase with glyphosate, and S‑2 wrongly links EPSPS with kanamycin.​

• EPSPS is not an aminoglycoside resistance enzyme; its connection is exclusively with glyphosate‑based herbicide resistance in engineered crops.​