Lac Operon Genes — i (Repressor), z (β-galactosidase), y (Permease), a (Transacetylase)
✅ Asked in NEET 2026In the lac operon, the z gene codes for
In the lac operon, the z gene codes for
The lactose present in the growth medium of bacteria is transported to the cell by the action of:
Given below are two statements: Statement I: In the lac operon, the z gene codes β-galactosidase which is primarily responsible for the hydrolysis of lactose into galactose and glucose. Statement II: In addition to lactose, glucose or galactose can also induce the lac operon.
In the lac operon, the i gene codes for:
Match List I with List II: (NEET 2023) List I List II A. gene ‘a’ I. β-galactosidase B. gene ‘y’ II. Transacetylase C. gene ‘i’ III. Permease D. gene ‘z’ IV. Repressor protein
In lac operon, z gene codes for: (NEET 2022 Phase 2)
Match the following genes of the Lac operon with their respective products: (NEET 2019) (A) i gene – (i) β-galactosidase (B) z gene – (ii) Permease (C) a gene – (iii) Repressor (D) y gene – (iv) Transacetylase
Answer & NCERT explanation
Correct answer: C — beta-galactosidase
In the lac operon, the z gene codes for beta-galactosidase, which hydrolyses lactose into galactose and glucose. The y gene codes for permease and a gene codes for transacetylase.
Read more NCERT concept on the PYQ
The lac operon consists of one regulatory gene (the i gene – here the term i does not refer to inducer, rather it is derived from the word inhibitor) and three structural genes (z, y, and a). The i gene codes for the repressor of the lac operon. The z gene codes for beta-galactosidase (β-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. The y gene codes for permease, which increases permeability of the cell to β-galactosides. The a gene encodes a transacetylase. Hence, all the three gene products in lac operon are required for metabolism of lactose. In most other operons as well, the genes present in the operon are needed together to function in the same or related metabolic pathway.
The LAC OPERON has ONE REGULATORY GENE (the i gene, where 'i' is derived from 'INHIBITOR', NOT 'inducer') and THREE STRUCTURAL GENES (z, y, and a). The i gene codes for the REPRESSOR of the operon. The z gene codes for BETA-GALACTOSIDASE (β-gal), which hydrolyses lactose into GLUCOSE + GALACTOSE. The y gene codes for PERMEASE, which transports lactose INTO the cell across the membrane (increases permeability to β-galactosides). The a gene encodes TRANSACETYLASE. All three structural-gene products are needed together for lactose metabolism. The natural INDUCER is LACTOSE (or its isomer allolactose) — when present, it binds the repressor and releases it from the operator, allowing transcription of z, y, a.
Lac operon is the HIGHEST-TESTED topic in this batch — NEET 2026, 2024 (×3), 2023, 2022, 2019 all asked it. Master the four gene-product pairs cold: i→repressor, z→β-galactosidase, y→permease, a→transacetylase. NEET 2024 trap: 'glucose or galactose can also induce the lac operon' — WRONG. Only LACTOSE (or allolactose) is the inducer; glucose REPRESSES the operon (catabolite repression); galactose alone does NOT induce. The 'i' in i-gene stands for INHIBITOR (NCERT explicit) — NOT inducer.
The 'i' in the lac operon i-gene stands for 'inducer', and glucose or galactose can also induce the operon.
'i' = INHIBITOR (NCERT exact). i gene codes for the REPRESSOR. Only LACTOSE/ALLOLACTOSE is the inducer. Glucose REPRESSES via catabolite repression; galactose alone does NOT induce.
i = INHIBITOR (repressor). z = β-gal (hydrolyses lactose). y = permease (transports lactose IN). a = transacetylase. Inducer = LACTOSE only.
- i gene = REPRESSOR (the 'i' stands for INHIBITOR, NOT inducer — NCERT explicit).
- z gene = β-GALACTOSIDASE (hydrolyses lactose → glucose + galactose).
- y gene = PERMEASE (transports lactose INTO the cell, increases membrane permeability to β-galactosides).
- a gene = TRANSACETYLASE. Inducer = LACTOSE/allolactose; glucose REPRESSES (catabolite repression); galactose alone does NOT induce.
Match the lac operon genes with their products: List I: A. gene 'a' B. gene 'y' C. gene 'i' D. gene 'z' List II: I. β-galactosidase II. Transacetylase III. Permease IV. Repressor protein
View bonus solution & explanation
Correct answer: B — A-II, B-III, C-IV, D-I
A-II: gene 'a' codes for TRANSACETYLASE. B-III: gene 'y' codes for PERMEASE (transports lactose INTO the cell). C-IV: gene 'i' codes for REPRESSOR protein ('i' = inhibitor, NCERT explicit — NOT inducer). D-I: gene 'z' codes for β-GALACTOSIDASE (hydrolyses lactose into glucose and galactose). The correct match is A-II, B-III, C-IV, D-I → option B (NEET 2023 answer). Memory rule: i-z-y-a → Repressor-β-gal-Permease-Transacetylase. Master this ordering cold — NEET has asked it 7 times in 5 years.
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