Lac Operon Quiz Diagram – Find Each Gene and Protein

All 14 parts on the diagram

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No lactose: switched off 8

Regulatory gene (lacI) It codes for the repressor. It lies just outside the operon, with a promoter of its own.
Promoter RNA polymerase binds here to start transcription. It lies just before the operator.
Operator The repressor binds to this stretch of DNA. It lies between the promoter and the genes.
lacZ The first structural gene. It codes for beta-galactosidase.
lacY The second structural gene. It codes for the permease.
lacA The third structural gene. It codes for the transacetylase.
Repressor A protein made by the regulatory gene. On the operator it keeps the genes switched off.
RNA polymerase The enzyme that transcribes the genes into mRNA. Here the repressor is in its way.

With lactose: switched on 6

Allolactose (inducer) A molecule the cell makes from lactose. It binds to the repressor and switches the operon on.
Inactive repressor The repressor with allolactose bound to it. Its shape has changed and it cannot hold the operator.
mRNA One long messenger RNA carries all three genes. The three proteins are made from it.
Beta-galactosidase The enzyme coded by lacZ. It splits lactose into glucose and galactose.
Lactose permease The protein coded by lacY. It sits in the cell membrane and brings lactose in.
Transacetylase It is the enzyme coded by lacA. Breaking down lactose needs only the other two.

How this list was compiled: Every name checked on 2026-10-01 against Wikipedia: Lac operon, text only; Kimball's Biology Pages: The Operon; Nature Education, Scitable: Operons and Prokaryotic Gene Regulation; the US National Human Genome Research Institute glossary (Promoter, Repressor); and, on 2026-10-02, four textbooks read on archive.org by full-text search. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; nothing traced. Licence: own work

How it works

On this lac operon quiz diagram a gene, a control site or a protein is named and you tap it on the chart. Eight parts are asked with no lactose, then six with it. What you miss is offered again after each chart.

Read the full guide

Pairs that get mixed up

The promoter is the box under RNA polymerase; the operator is the short box after it, under the repressor. A gene is a box on the DNA and its protein is a shape under the mRNA, so lacZ and beta-galactosidase are two answers. On the second chart the protein that has left the DNA, with allolactose in its top, is the inactive repressor.

Read the DNA left to right

The regulatory gene lacI stands alone at the left end. Then come the promoter, the operator and the structural genes in the order Z, Y, A, with lacZ drawn longest. The three proteins stand under their genes in the same order.

The I of lacI stands for inducibility.

Questions and answers

Is the lac operon inducible or repressible?

Inducible. Its genes stay off until an inducer, allolactose, takes the repressor off the operator. Because the switch is a repressor, the control is called negative inducible.

Is lacI part of the lac operon?

No. lacI lies beside the operon with a promoter of its own and is expressed all the time, so the cell always holds some repressor.

What does polycistronic mean for the lac operon?

One transcript that codes for several proteins. lacZ, lacY and lacA share one promoter and are copied into a single mRNA, so the three are switched on and off together.