The lac operon and transcription factors
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 4 practice questions, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Genes that switch on and off
- An operon
- No lactose
- Lactose present, no glucose
- A past-paper example
Read the transcript of the teaching part
Last time
Before we start: last time, in The chi-squared test and the gibberellin gene, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Genes that switch on and off
Every cell in your body has the same D N A, yet a muscle cell and a nerve cell are very different. Genes are switched on in one and off in the other. Bacteria do the same thing to save energy: they make an enzyme only when its sugar is present. The lac operon is the classic example, and transcription factors are the eukaryote version.
An operon
An operon is a group of genes in a prokaryote that are controlled together. They have a shared promoter and operator, and are transcribed as one mRNA. The lac operon has three structural genes. lacZ codes for beta galactosidase, which hydrolyses lactose. lacY codes for the permease that brings lactose into the cell. lacA codes for a transacetylase. A separate regulatory gene, lacI, codes for the repressor protein. The promoter is where R N A polymerase binds, and the operator is where the repressor binds.
No lactose
The repressor protein is made all the time by the regulatory gene. It has a shape that fits the operator. When it binds to the operator, it blocks the promoter, and R N A polymerase cannot start transcription. The structural genes are not transcribed. So the enzymes are not made. The cell saves energy and materials.
Lactose present, no glucose
Lactose, or its isomer allolactose, binds to the repressor protein. It changes the shape of the repressor, so that it no longer fits the operator. The repressor leaves the operator. R N A polymerase can bind to the promoter and transcribe the structural genes. So the enzymes are made when lactose is present. They are inducible enzymes: they are only made when the substrate is there to induce them.
A past-paper example
May June 2025, Paper 43, question 3 part a. Explain why the enzymes coded for by the lac operon are described as inducible enzymes. Two marks. They are made only when the substrate, lactose, is present. Lactose induces transcription by binding to the repressor. Without lactose, the genes are off.
Chapter 16: Inheritance
- Meiosis and how genes make phenotypes 5 min
- Monohybrid crosses, codominance and sex linkage 5 min
- Dihybrid crosses, linkage and epistasis 6 min
- The chi-squared test and the gibberellin gene 5 min
- The lac operon and transcription factors 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.