Regulation of transcription (transcription factors)

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. 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 and decide whether RNA polymerase is allowed to start reading the gene.

Transcription factor present (right kind) → gene ON. Absent or blocked → gene OFF.

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.

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.

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.

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.

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.

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.

How this is tested: HL questions on D2.2 ask you to explain how transcription factors control gene expression. Score points for: transcription is the main control point; a transcription factor is a protein that binds a regulatory sequence; an activator helps RNA polymerase bind the promoter → gene ON; a repressor blocks it → gene OFF; and so the set of transcription factors present decides which genes are transcribed.

A higher-mark version adds the consequence: this is why different cell types express different genes from the same DNA, and how a cell responds to signals (e.g. a hormone acting as/through a transcription factor).

✓ Why this scores full marks: It names transcription as the control point, defines a transcription factor as a protein binding a regulatory sequence, gives both outcomes (activator → ON, repressor → OFF) with the RNA polymerase / promoter mechanism, and finishes with the consequence (the set of factors present decides which genes are read, allowing responses to signals).

A common way to lose marks is to describe only activation — full marks usually need the repressor (gene-OFF) side too, plus the 'which factors → which genes' conclusion.