Lac Operon
லாக் ஓபரான்
Stop memorizing diagrams and start understanding metabolic economy. Bacteria do not waste ATP synthesizing enzymes for lactose when glucose is available. This is the fundamental principle of gene regulation in prokaryotes.
The idea
The lac operon consists of one regulatory gene (i) and three structural genes (z, y, a). The i-gene produces a repressor protein that binds to the operator, blocking RNA polymerase. When lactose acts as an inducer, it binds to the repressor, inactivating it and allowing transcription of the structural genes to proceed.
Why it matters
It allows bacteria to conserve energy by producing enzymes for lactose metabolism only when lactose is present in the environment.
NEET lens
NEET focuses on the role of the i-gene and the function of the inducer. Expect questions on the 'z', 'y', and 'a' gene products: beta-galactosidase, permease, and transacetylase. Do not confuse the operator with the promoter; the repressor binds to the operator, not the promoter.
Clinical significance
While the lac operon is prokaryotic, the principle of inducible gene expression is critical in human pharmacology. Many antibiotics function by targeting bacterial protein synthesis pathways, exploiting these regulatory mechanisms. Failure in human gene regulation leads to oncogenesis, where genes that should be 'off' are permanently 'on'.
Introduction
The lac operon in E. coli is a set of genes for lactose metabolism that are switched on only when lactose is present. It was the first operon described, by Jacob and Monod.
Core concept
Parts: regulatory gene i (codes for the repressor; 'i' from inhibitor), promoter, operator, and three structural genes — z (beta-galactosidase: lactose → glucose + galactose), y (permease: increases cell permeability to lactose) and a (transacetylase). The repressor is made all the time (constitutively). No lactose: repressor binds the operator → RNA polymerase cannot transcribe → operon off. Lactose present: lactose (inducer; actually allolactose) binds the repressor → repressor inactivated, cannot bind operator → RNA polymerase transcribes z, y, a → operon on. This is negative regulation by the repressor; the lac operon is an inducible operon.
Go deeper
Regulation of the lac operon by the repressor is called negative regulation. NCERT notes the lac operon is also under positive regulation, but that is beyond the syllabus. The repressor is always made from the i gene; it is lactose that decides whether it works.
From Class 10
Cells do not make every protein all the time. Like switching off a light when you leave a room, bacteria switch genes off when they are not needed.