The big idea: Every cell in your body carries the same DNA, yet a liver cell and a nerve cell make different proteins. How? Cells control which genes are switched on.
Transcription is the switch
- The main control point is transcription — deciding whether a gene is even copied into mRNA in the first place.
- The switches are proteins called transcription factors.
- They bind to special regulatory sequences in the DNA.
- They decide whether RNA polymerase is allowed to start reading the gene.
On or off: Transcription factor present (right kind) → gene ON. Absent or blocked → gene OFF.
Gene ON (top panel): an activator transcription factor binds the regulatory sequence, which helps RNA polymerase settle on the promoter and transcribe the gene → mRNA is made. The lower panel previews the OFF state (covered in the next micro).
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- Gene expression
- Switching a gene on so its information is used — the gene is transcribed (and then usually translated into a protein).
- Transcription
- Copying a gene's DNA into a strand of mRNA, carried out by RNA polymerase. This is the MAIN point at which genes are switched on or off.
- Transcription factor
- A protein that binds a regulatory DNA sequence and controls whether a gene is transcribed.
- Regulatory sequence
- A specific stretch of DNA (such as a promoter or enhancer) that transcription factors bind to, controlling a nearby gene.
- Promoter
- The regulatory sequence just before a gene where RNA polymerase must bind to begin transcription.
- RNA polymerase
- The enzyme that binds the promoter and transcribes the gene into mRNA.
Why control transcription, not later steps?: A cell could block a gene's product after the mRNA or protein is made — but that wastes energy building things it will throw away.
Controlling it at transcription is the most efficient point: if the gene is never copied to mRNA, nothing downstream is made. That is why transcription is the main switch.
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There are two kinds of transcription factor, named by what they do.
Read each as a chain of cause and effect: the protein binds a regulatory sequence, and that binding either helps or blocks RNA polymerase at the promoter — which switches the gene on or off.
ACTIVATOR → gene ON
- An activator (a transcription factor) is present in the cell.
- It binds a regulatory sequence in the DNA (e.g. the promoter or an enhancer).
- This helps RNA polymerase bind the promoter.
- RNA polymerase starts transcription and reads the gene.
- mRNA is made — the gene is switched ON (expressed).
Follow the top panel: transcription factor bound → RNA polymerase on the promoter → transcription → mRNA produced. No transcription factor (or a repressor in the way) = no RNA polymerase = no mRNA.
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REPRESSOR → gene OFF
- A repressor (a transcription factor) is present in the cell.
- It binds a regulatory sequence at or near the promoter.
- This blocks RNA polymerase from binding the promoter.
- Transcription cannot start.
- No mRNA is made — the gene is switched OFF.
Same enzyme, opposite outcome: Notice that RNA polymerase is the same in both cases. What differs is the transcription factor sitting on the DNA:
An activator helps the polymerase on → gene ON.
A repressor blocks the polymerase → gene OFF.
So a gene's state is decided by which transcription factors are bound to its regulatory sequences.
| Activator | Repressor | |
|---|---|---|
| What it is | A transcription factor (a protein) | A transcription factor (a protein) |
| Where it binds | A regulatory sequence in the DNA (e.g. promoter / enhancer) | A regulatory sequence in the DNA (often at or near the promoter) |
| Effect on RNA polymerase | HELPS RNA polymerase bind the promoter | BLOCKS RNA polymerase / the promoter |
| Effect on transcription | Transcription STARTS | Transcription is PREVENTED |
| The gene is therefore | Switched ON (mRNA is made) | Switched OFF (no mRNA made) |
Why different cells express different genes: Because the DNA is the same in every cell, the difference between cell types comes from which transcription factors each cell makes.
One genome, many cell types
- A liver cell contains the activators that switch on liver genes.
- A nerve cell contains a different set that switches on nerve genes.
- So the set of transcription factors present in a cell determines which genes are transcribed — this is what makes a liver cell a liver cell.
Responding to a signal: hormones: Transcription-factor control also lets a cell respond to signals.
How a steroid hormone switches genes on
- A steroid hormone such as oestrogen can pass into a target cell and bind a receptor.
- The hormone–receptor complex then acts as a transcription factor.
- It binds regulatory sequences and switches specific genes on.
- So a signal arriving at the cell can change which genes are transcribed, changing what the cell does.
- The same logic underlies a bacterium switching on sugar-digesting genes only when that sugar is present.
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The big idea: Transcription is controlled by transcription factors: proteins that bind to specific base sequences in the DNA, at the promoter and at enhancers.
The points to remember
- The promoter is a base sequence just before a gene, where RNA polymerase binds to start transcription.
- Transcription factors are proteins that bind to specific base sequences in the DNA.
- Some transcription factors help RNA polymerase bind to the promoter.
- An enhancer is a base sequence, often far from the gene, where an activator binds to increase transcription.
- A repressor binds the DNA and blocks transcription, switching the gene off.
Remember it as: Promoter: where to start. Enhancer: how fast.
Real example: lactase in adults. In many people of European descent, a change in an enhancer far upstream of the lactase gene keeps the gene transcribed in adulthood, so they can still digest milk sugar as adults.
How it is asked: A long Paper 2 answer on how gene expression is regulated at transcription (promoters, enhancers, transcription factors, and epigenetic tags), or short items on what a transcription factor does.
Explain how transcription factors regulate the transcription of a gene.
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For a seven-mark answer: Add the other controls too: methylation of the promoter and of histones (4.5.6), hormones (4.5.11), and how the protein made gives the phenotype.