The big idea: A gene is just a set of instructions in DNA. On its own it does nothing — it must be expressed.
When a gene is expressed
Transcribed
The gene is copied into mRNA.
Translated
The mRNA is read by a ribosome.
Protein
The cell builds the protein that the gene codes for.
Gene expression = transcription + translation, ending in the gene's protein product.
A switched-off gene: 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 expression = transcription + translation. A gene is 'expressed' when its DNA is transcribed into mRNA and that mRNA is translated into a protein. A gene that is NOT expressed is never copied into mRNA, so its protein is never made.
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- 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).
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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.
Why it can't be different genes
- Every body cell came from one zygote by mitosis.
- Mitosis 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.
Why that changes the cell: 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.
The cause → effect chain
- Every cell carries the same DNA (the same full genome).
- Each cell type switches on a different subset of genes (differential expression).
- The switched-on genes are transcribed and translated into proteins.
- So each cell makes a different mix of proteins.
- Those proteins build and run the cell → different structure and function.
| Step | What happens | Why cells end up different |
|---|---|---|
| 1. Same DNA | Mitosis copies the whole genome into every body cell | Difference is NOT in the DNA — all cells share it |
| 2. Differential expression | Each cell type switches ON only a subset of its genes | Different genes are transcribed in different cells |
| 3. Transcription + translation | The switched-on genes are made into mRNA, then protein | Each cell makes a different MIX of proteins |
| 4. Structure & function | Those proteins build and run the cell | Neuron ≠ liver cell, even with identical DNA |
| Neuron | Liver cell | |
|---|---|---|
| Genes switched on | Ion channels and neurotransmitter enzymes | Detox enzymes and glycogen handling |
| Genes kept off | — | Ion-channel and neurotransmitter genes |
| What it is built for | Carrying electrical signals | Processing the blood |
Worked example — neuron vs liver cell: 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.
| Neuron (nerve cell) | Hepatocyte (liver cell) | |
|---|---|---|
| Genome (DNA) | The SAME full set of genes | The SAME full set of genes — identical to the neuron's |
| Genes switched ON | Genes for ion channels, neurotransmitter enzymes, signalling proteins | Genes for detox enzymes, glycogen-handling enzymes, blood-protein production |
| Genes switched OFF | Most liver-specific genes are silent | Most neuron-specific genes are silent |
| Proteins actually made | Mostly nerve-signalling proteins | Mostly liver-metabolism proteins |
| Result | A long, branched cell that carries electrical signals | A cube-shaped cell that filters blood and stores glucose |
Two cells with identical DNA differ because they run this pathway for DIFFERENT genes. A neuron expresses its set of genes; a liver cell expresses a different set — so each ends up with a different mix of proteins, structure and function.
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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.
What that means
- That pattern is kept up over the cell's life.
- It is 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.
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The big idea: Gene expression is how the information in a gene has an effect on the phenotype: the gene is transcribed, the mRNA is translated, and the protein made does a job, often as an enzyme.
The points to remember
- Transcription: the gene's base sequence is copied into mRNA.
- Translation: ribosomes use the mRNA to join amino acids into a polypeptide.
- The polypeptide folds into a protein with a function, such as an enzyme.
- What the protein does produces a characteristic: part of the phenotype.
- A gene that is not expressed has no effect on the phenotype.
Remember it as: Gene, mRNA, protein, trait.
Real example: albinism. The gene for tyrosinase is needed to make melanin. In people with one kind of albinism, the gene does not give a working enzyme, so little melanin is made and hair, skin and eyes are very pale.
How it is asked: Often as data: expression levels of genes after a treatment (outline the effect on each gene), then a longer answer linking a gene's expression to a trait.
Explain how the information in a gene can affect the phenotype of an organism.
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Read the error bars: If the error bars of two bars overlap, say there is no significant difference in expression.