Gene expression and why cells differ

The big idea: A gene is just a set of instructions in DNA. On its own it does nothing.

A gene is expressed when the cell actually uses it — it is transcribed (copied into mRNA) and translated (read by a ribosome) to build the protein that the gene codes for.

So gene expression = transcription + translation, ending in the gene's protein product.

A gene that is switched off is never transcribed, so no mRNA and no protein are made — even though the gene is still sitting in the DNA.

Gene

A length of DNA that codes for one protein (the gene's product).

Gene expression

The transcription and translation of a gene to make its protein. A gene is 'expressed' when its protein is being produced.

Gene product

The protein that a gene codes for — the end result of expressing that gene.

Switched on / off

An expressed gene is 'switched on'; a gene that is not transcribed is 'switched off' (silent), so its protein is not made.

Genome

The complete set of genes (all the DNA) in a cell.

Having a gene ≠ using a gene: Every cell has thousands of genes, but only uses some of them at any time.

Think of the genome as a cookbook and expression as actually cooking a recipe. Owning the recipe does nothing — the dish only appears when you read it and cook (transcribe and translate).

Here is the puzzle. A neuron in your brain and a cell in your liver look and behave completely differently — yet they contain exactly the same DNA.

Every body cell came from one zygote by mitosis, which copies the whole genome faithfully. So the difference between cell types cannot come from different genes — they all carry the same set.

The answer: differential gene expression: Cells differ because they express different genes.

Out of the same full genome, each cell type switches on only a subset of its genes and leaves the rest off. This is called differential gene expression.

Because different genes are switched on, each cell type transcribes and translates a different set of proteins — and it is the proteins that give a cell its structure and function.

Worked example — neuron vs liver cell: A neuron switches on genes for ion channels and neurotransmitter enzymes, so it is built to carry electrical signals.

A liver cell keeps those genes off but switches on genes for detox enzymes and glycogen handling, so it is built to process the blood.

Same DNA, different genes expressed → different proteins → different cell. A red-blood-cell precursor makes the difference obvious: it switches on the haemoglobin gene (which stays off in a neuron), so it fills with haemoglobin while the neuron never does.

Differentiation = a stable expression pattern: When a cell differentiates (becomes a neuron, a liver cell, a muscle cell), it settles into a stable pattern of which genes are switched on.

That pattern is kept up over the cell's life and usually passed to its daughter cells, which is why a liver cell keeps making liver-cell daughters.

So a 'cell type' is really just a particular set of expressed genes.

How this is tested: The classic HL question gives you two very different cells with identical DNA and asks you to explain how they can differ.

The marks live in the chain: same genome in every cell → each expresses a different subset of genes (differential gene expression) → genes are transcribed and translated into different proteins → different structure and function.

Finish with the key term — gene expression = transcription + translation — and the idea that differentiation is a stable pattern of which genes are switched on.

✓ Why this scores full marks: It nails the paradox — same DNA but different genes expressed — and follows the chain all the way to different proteins → different cell.

A common way to lose marks is to say the cells have different genes or lose genes. They don't — every cell keeps the whole genome; only which genes are switched on differs.