• 14 parts
  • Checked October 2026

Lac Operon Diagram

The same strip of DNA drawn twice: switched off with no lactose, and switched on when lactose arrives.

Lac Operon Diagram

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Write the name of each numbered part.Answer key

  1. Regulatory gene (lacI)
  2. Promoter
  3. Operator
  4. lacZ
  5. lacY
  6. lacA
  7. Repressor
  8. RNA polymerase
  9. Allolactose (inducer)
  10. Inactive repressor
  11. mRNA
  12. Beta-galactosidase
  13. Lactose permease
  14. Transacetylase

Drawn for this site on 2026-10-01 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: RNA polymerase at work

All 14 parts on the diagram

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No lactose: switched off 8

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

With lactose: switched on 6

  1. Allolactose (inducer) A molecule the cell makes from lactose. It binds to the repressor and switches the operon on.
  2. Inactive repressor The repressor with allolactose bound to it. Its shape has changed and it cannot hold the operator.
  3. mRNA One long messenger RNA carries all three genes. The three proteins are made from it.
  4. Beta-galactosidase The enzyme coded by lacZ. It splits lactose into glucose and galactose.
  5. Lactose permease The protein coded by lacY. It sits in the cell membrane and brings lactose in.
  6. 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

The lac operon is a set of three genes in the bacterium E. coli that are switched on together when lactose is present. Label the structures of the lac operon on the first chart, then see what lactose changes.

Read the full guide

No lactose: the operon is off

The regulatory gene, lacI, makes a repressor protein. The repressor sits on the operator, next to the promoter where RNA polymerase binds. With the repressor in the way, the three genes lacZ, lacY and lacA are not transcribed.

Lactose present: the operon is on

Allolactose, made from lactose, binds to the repressor and changes its shape, so it leaves the operator. RNA polymerase then transcribes all three genes into one mRNA, and three proteins are made from it.

Questions and answers

What are the parts of the lac operon?

A promoter, an operator and three structural genes: lacZ, lacY and lacA. The regulatory gene lacI lies just outside it and makes the repressor.

What does the lac repressor do?

With no lactose, it binds to the operator and keeps RNA polymerase from transcribing the genes. When allolactose binds to it, it changes shape and lets go of the DNA.

What do lacZ, lacY and lacA code for?

lacZ codes for beta-galactosidase, which splits lactose into glucose and galactose. lacY codes for a permease that brings lactose into the cell. lacA codes for a transacetylase.

What is the inducer of the lac operon?

Allolactose, a molecule the cell makes from lactose. It binds to the repressor, so the operon is switched on only when lactose is there to be used.

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