A nerve cell and a white blood cell contain exactly the same DNA. Every gene for both is present in both. The difference is which genes are switched on — and once you see that, specialisation stops being mysterious.
📚 What you need to know
Every cell in an organism carries the same genome, because all of them came from one fertilised egg.
Differentiation is a cell becoming specialised by expressing only some of its genes.
Which genes are expressed decides which proteins are made, and the proteins decide the cell’s structure and job.
Stem cells are unspecialised cells that can divide repeatedly and then differentiate.
Stem cells are described by their potency: totipotent, pluripotent, multipotent, unipotent.
Stem cells sit in niches such as bone marrow, and signals in the niche control what they become.
They can be used to treat disease, for example replacing blood cells in bone marrow transplants.
Same instructions, different pages
Think of the genome as a very thick recipe book that every cell owns a copy of. A muscle cell reads the muscle pages. A pancreas cell reads the insulin page. Nobody tears out the pages they are not using — they just never open them.
Follow the chain in that order: genes expressed → proteins made → structure and function. Almost every differentiation question is asking you to walk along it.
How it actually works
Every body cell came from the same zygote by mitosis, so every one has the same genes.
In any given cell only some of those genes are expressed — transcribed into mRNA and translated into protein.
The set of proteins produced determines the cell’s structure and what it can do.
Chemical signals from neighbouring cells and from the cell’s position in the embryo decide which genes get switched on.
Once a cell has differentiated it usually stays that way permanently.
Where the evidence comes from. Cloning experiments showed that a nucleus taken from a fully differentiated adult cell can, in the right conditions, produce a whole new organism. That only works if the nucleus still contains every gene. So the genes are not lost during differentiation — they are just kept quiet.
Stem cells and potency
A stem cell is unspecialised, can keep dividing, and can differentiate into other cell types. How many types it can become is its potency, and the four terms form a ladder from most flexible to least.
The prefixes do the work: toti means all, pluri means many, multi means several, uni means one.
Type
What it can become
Where it is found
Totipotent
Every cell type in the organism, plus the placenta
The zygote and the cells of the very early embryo
Pluripotent
Any cell type in the body, but not the placenta
Embryonic stem cells in the blastocyst
Multipotent
A limited range of related cell types
Bone marrow, where they form the different blood cells
Unipotent
Only one cell type, though it keeps dividing
Skin, and the cells that repair heart muscle
🧠
Count down the prefixes
Toti = total, pluri = plural, multi = many, uni = one. The order is the same as the number of options, so the list sorts itself.
Stem cell niches
Stem cells are not scattered at random. They sit in particular places called niches, where the surrounding cells send signals that keep them unspecialised until they are needed.
Bone marrow holds multipotent stem cells that produce red blood cells, white blood cells and platelets throughout life.
Umbilical cord blood is a rich source of stem cells and can be collected at birth and stored.
The base of hair follicles and the lower layer of skin hold stem cells that constantly replace worn cells.
Bone marrow transplants are the everyday example worth knowing. Donated marrow contains multipotent stem cells that settle in the patient’s bones and start producing healthy blood cells. It is used for leukaemia, and it works precisely because those cells have not yet differentiated.
Worked examples
WE 1
Explain how differentiation happens
All cells in an organism contain the same genes. Explain how they can have different structures and functions. (3 marks)
Point 1: the genes are all there
Every cell came from the same zygote by mitosis, so all carry the same genome.
Point 2: expression differs
In each cell only some genes are expressed, so only some proteins are made.
Point 3: the consequence
The proteins a cell makes determine its structure and what it can do, so cells with different genes switched on become different cell types.
Same genes, different genes expressed, different proteins madethe word “expressed” is what the mark scheme looks for — not “used” or “activated”
WE 2
Distinguish two kinds of stem cell
Distinguish between totipotent and multipotent stem cells. (2 marks)
Difference 1: what they form
Totipotent cells can form any cell type including placental tissue, while multipotent cells form only a limited range of related types.
Difference 2: where they are
Totipotent cells are found in the zygote and very early embryo; multipotent cells are found in adult tissues such as bone marrow.
Totipotent forms everything; multipotent forms a narrow group“distinguish” needs both sides in the same sentence, not two separate descriptions
WE 3
Apply it to a treatment
Explain why bone marrow stem cells can be used to treat a patient whose blood cells are not being produced properly. (3 marks)
Point 1: they are unspecialised
Bone marrow stem cells are multipotent and have not yet differentiated.
Point 2: what they can do
They can divide repeatedly and differentiate into red blood cells, white blood cells and platelets.
Point 3: the effect on the patient
Once transplanted they settle in the bone marrow and produce healthy blood cells, replacing the faulty supply.
Unspecialised, still dividing, and able to become every type of blood cellsay multipotent rather than just “stem cells” — the precision is usually worth a mark
💡 Exam tips
Use the phrase gene expression. Saying genes are “turned on” alone is often not enough.
Follow the chain: genes expressed, proteins made, structure and function.
Learn the four potency terms with one example of each.
Say unspecialised and able to divide repeatedly when defining a stem cell — both halves are needed.
Bone marrow is the safest example to give for a therapeutic use.
Remember that no genes are lost during differentiation.
⚠ Common mistakes
Saying cells lose the genes they do not need. They keep all of them.
Confusing pluripotent and totipotent. Only totipotent cells can form placental tissue.
Calling all stem cells embryonic. Adults have them too, in bone marrow and skin.
Saying a stem cell is one that divides. It must also be unspecialised.
Writing that differentiation is reversible in the body. It is normally permanent once complete.
Skipping the protein step. Genes do not build a cell directly; proteins do.
Up next: Multicellularity — what an organism gains by having specialised cells, and the levels of organisation built on top of them.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
