In the lac operon how does the inducer actually switch it on? Plus is DNA fingerprinting based on VNTRs?
Zoya Khan12th Sample
10 Oct 2026 · 1 min read
Revising the last part of Molecular Basis of Inheritance and I want to make sure I actually understand the lac operon instead of just memorising the diagram.
My understanding: the i gene makes a repressor. When lactose is absent the repressor binds to the operator, so RNA polymerase can't transcribe the structural genes z, y, a. When lactose is present, it's called the inducer and something happens to the repressor. But I'm not sure what: does the inducer bind to the operator, or to the repressor, or to the promoter? NCERT's wording "the repressor is inactivated by interaction with the inducer" feels slippery. Also why is this called negative regulation?
Also two quick fact-checks. HGP: about 3 × 10^9 base pairs, and less than 2 per cent of the genome codes for proteins. DNA fingerprinting works by finding VNTRs (variable number of tandem repeats), which differ from person to person, using Southern blotting and hybridisation with a probe. Did I get the main points right?
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[+][–]Kavya NaikDropperSample· 9d ago
The inducer binds the repressor protein, not the DNA. Lactose (really its isomer allolactose in the cell, but NCERT just says lactose) attaches to the repressor and changes its shape, so it can no longer stick to the operator. Operator free, so RNA polymerase moves ahead, transcribes the z, y, a genes as one mRNA, and the cell makes the enzymes for lactose use. When lactose is gone, repressor goes back to normal and shuts it again.
[+][–]Zoya Khan12thSample· 9d agoAuthor
oh it's the repressor that changes. That's clear now, thank you. I was imagining the lactose sitting on the DNA like a key.
[+][–]Kavya NaikDropperSample· 9d ago
haha yes, "inducer = the key that unlocks the brake, not the key to the road". Hope it sticks!
[+][–]Rahul DubeyDropperSample· 9d ago
Negative regulation = the regulatory protein stops transcription. The repressor is the "brake". The inducer is just what releases the brake. In positive regulation the regulator protein must bind to help switch a gene on. NCERT's phrase is "regulation of lac operon by repressor is referred to as negative regulation".
The three structural genes: z codes for beta-galactosidase (breaks lactose into galactose and glucose), y codes for permease (lets lactose into the cell), a codes for transacetylase.
[+][–]Pooja Iyer12thSample· 9d ago
I once wrote that the inducer attaches to the operator and my teacher drew a huge red circle around it, so I'll save you: it's the repressor that sees the inducer. The promoter is where RNA polymerase sits. The operator is where repressor sits. Order on the DNA in the usual diagram: promoter, operator, then z, y, a. (The i gene has its own promoter and sits just before.)
[+][–]Riya Singh12thSample· 9d ago
Your HGP facts are on track. NCERT also says 99.9% of nucleotide bases are exactly the same in all people, and that functions are unknown for over half of the discovered genes. SNPs (single nucleotide polymorphisms) were found at about 1.4 million locations, useful for tracing history and finding disease associated sequences. One more: repetitive sequences make up very large portions of the human genome.
[+][–]Ishita GoyalDropperSample· 9d ago
Fingerprinting, step by step, because the order is asked a lot: isolate DNA, cut it with restriction enzymes, separate fragments by gel electrophoresis, transfer to a nylon membrane (Southern blotting), hybridise with a labelled VNTR probe, then detect by autoradiography. The bands differ between people since the number of VNTR repeats differs, but identical twins show the same pattern. And since you inherit half from each parent, it's used in paternity tests too.
