Your DNA is a recipe book, not a meal. Nothing happens until a cell actually reads a recipe and cooks something from it. That reading-and-cooking is what we call gene expression — and it is the reason two cells with exactly the same DNA can end up looking and behaving completely differently.
📚 What you need to know
Gene expression is the process by which the code stored in DNA ends up affecting the phenotype of an organism.
The usual stages are transcription (DNA copied into mRNA), translation (mRNA read to build a protein), and then the protein doing its job.
Transcription is the main stage where a gene gets switched on or off.
Every body cell in an organism carries the same genes, but each cell type expresses a different selection of them.
The protein made is what you finally see as a characteristic — an enzyme, a pigment, a receptor.
Replication copies DNA for cell division. It is not part of gene expression, so do not list it as a stage.
What gene expression actually means
A gene is just a length of DNA with a base sequence. On its own it does nothing at all. For that sequence to matter, the cell has to copy it, read the copy, and build a protein from it. Only then does the gene have any effect on the organism.
So when a question asks you to define gene expression, it wants that whole route: from the code in DNA to a feature you can actually observe. Everything in this topic hangs off that one sentence.
The route in one line
DNA (gene) → mRNA → protein → phenotype
Each arrow is a place the cell can speed things up or shut them down — but the first arrow is the one it uses most.
The three stages, one at a time
1. Transcription
An enzyme called RNA polymerase copies the base sequence of one gene into a molecule of mRNA. The DNA stays safely in the nucleus; the copy is what travels. This is the big control point: if the copy is never made, the protein is never made, no matter how perfect the gene is.
2. Translation
The mRNA moves out to a ribosome in the cytoplasm. The ribosome reads the mRNA three bases at a time and joins the matching amino acids together into a polypeptide chain.
3. The protein does a job
This is the stage students forget, and it is worth a mark on its own. A protein only changes the phenotype once it starts working — catalysing a reaction as an enzyme, carrying oxygen, sitting in a membrane as a receptor, or colouring a petal. No working protein, no visible characteristic.
Read the command term carefully. “Outline the stages of gene expression” wants transcription, translation and protein function. If you stop after translation you have described protein synthesis, not gene expression, and you lose the last mark.
Why cells do not express every gene
A liver cell and a nerve cell in the same person carry an identical set of genes. They look nothing alike because they read different parts of that set. A gene that is being transcribed is described as switched on; one that is not is switched off.
There are two good reasons for this. First, a cell only needs the proteins for its own job — a nerve cell has no use for digestive enzymes. Second, transcription and translation cost a lot of energy and raw materials, so expressing everything all the time would be enormously wasteful.
Switching a gene off does not delete it. The gene is still there in every cell — it is simply never copied.
Watch your wording. A gene that is “switched off” has not been removed, damaged or mutated. It is present and perfectly normal; the cell just is not transcribing it. Examiners are strict about this.
Worked examples
WORKED EXAMPLE
Outline what is meant by gene expression. [3]
Start with the definition
Gene expression is the way the code held in DNA affects the phenotype of an organism.
Then give the stages in order
The gene is transcribed into mRNA; the mRNA is translated into a protein at a ribosome; the protein then carries out a function, such as acting as an enzyme.
3 marks: definition + transcription/translation + protein functionNaming the final protein function is the mark most students drop.
WORKED EXAMPLE
A liver cell and a nerve cell from the same person contain identical DNA, yet they contain different proteins. Explain why. [3]
Point 1 — same genes, different expression
Both cells carry the same genes, but only some of those genes are transcribed in each cell type.
Point 2 — link to proteins
Only the genes that are transcribed produce mRNA, so only those are translated into protein.
Point 3 — why it is useful
Each cell only makes the proteins it needs for its own role, which avoids wasting energy and materials.
3 marks“Different genes are switched on” alone is 1 mark. Push it through to protein for full credit.
💡 Exam tip
Learn the one-line route DNA → mRNA → protein → phenotype and write it out at the top of your rough working. It structures almost every answer in this sub-topic.
Always finish with the protein doing something. Marks are given for the function, not just for making the protein.
Use “transcribed” and “translated” precisely. They are not interchangeable and examiners will not guess what you meant.
If a question mentions two different cell types, it is asking about differential gene expression — same genome, different genes read.
Keep replication out of your answer unless the question actually asks about cell division.
“Phenotype” means any observable feature, including things you cannot see with your eyes, such as blood group or an enzyme being present.
⚠ Common mix-up
Gene expression is not the same as protein synthesis. Expression includes what the protein then does.
Switched off does not mean absent. The gene is still in the DNA of every cell.
Transcription happens in the nucleus, translation in the cytoplasm. Mixing these up costs easy marks.
Genotype does not change when expression changes. The DNA sequence stays exactly as it was.
Not every gene codes for a protein. Some produce functional RNA instead — more on that in the next page.
mRNA is the copy, not the original. Never write that DNA leaves the nucleus.
Up next: Regulating Transcription & Translation — the promoters, enhancers and transcription factors that decide which genes get read in the first place.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
