What you need to know
- 3.18Differential geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. expression, active messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. and proteins in animal and plant specialisation.
By the end, you should be able to…
- Explain differentiationThe process by which a cell becomes specialised, because some genes are switched on and others are switched off. through gene expression.
- Link proteins to specialised structure and function.
- Distinguish presence of a gene from expression of that gene.
Build your understanding
Same genome, different activity
differentiationThe process by which a cell becomes specialised, because some genes are switched on and others are switched off. is the development of specialised cell structure and function. Most nucleated somatic cells of one organism contain essentially the same geneA sequence of DNA bases that codes for a polypeptide or a functional RNA., but different subsets are expressed. A cell does not normally become specialised by discarding all the genes it no longer needs.
The qualification matters: mature mammalian red blood cells lose their nuclei, gameteA haploid sex cell, such as a sperm or an egg cell. are haploidHaving one set of chromosomes (n), as in gametes. and mutationA change in the sequence of bases in DNA. can create differences. These exceptions do not replace differential gene expression as the main explanation of specialisation.
From an active gene to a useful protein
When a protein-coding gene is expressed, transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus. produces RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis.. Processing produces mature messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. that can be translated by ribosomeA small organelle made of rRNA and protein, where translation takes place.. The resulting polypeptideA chain of many amino acids joined by peptide bonds. folds and may be modified to become a functional protein. Different patterns of expression produce different sets or amounts of proteins.
Proteins can be enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. controlling metabolic reactions, receptors responding to signals, membrane transporters or structural components. Thus gene expression changes cell processes as well as shape. External signals and regulatory proteins can alter which genes are transcribed.
Animal and plant examples
An insulin-producing pancreatic beta cell expresses the insulin gene and has the secretory machinery needed to make and release its protein product. A muscle cell produces abundant contractile proteins. Both express many shared genes needed for basic metabolism, so specialisation does not mean that every expressed gene is unique to one cell type.
During development of a root hair cell, expression of genes for membrane transport proteins and proteins involved in cell-wall growth contributes to its uptake function and extended shape. Different expression patterns in plant cells therefore produce different structures and metabolic capabilities, just as they do in animals.
Use evidence at the right level
Finding a gene in DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. does not prove that it is being expressed. Detecting its mature mRNA supports transcription and processing, but protein amount also depends on translationMaking a polypeptide on a ribosome, using the sequence of codons on mRNA. tRNAs bring the amino acids. and breakdown. Detecting a protein does not automatically prove that it is active.
In a fictional comparison, two cell types both contain gene X, but only one contains abundant X mRNA and protein. This supports differential expression rather than loss of the gene from the other cell.
NOT: differentiation is not the loss of every unused gene. A red blood cell losing its nucleus is an exception.
- Present is not the same as expressed
- Proteins may be enzymes, receptors, transporters or fibres
- Plant cells specialise by the same principle
- Gametes are haploid, so they are not the usual example
Two traps this diagram blocks
- Do not say a muscle cell has deleted the genes for digestive enzymes.
- Do not treat every specialised cell as having lost its nucleus.
From the diagram alone
Question
Explain how two cells with the same genes can have different structures. [3]
Show a model answer
Different genes are expressed in each cell. Transcription and translation therefore produce different proteins. Those proteins determine the cell’s structure and function, so the cells specialise without losing the unused genes.
Make your reasoning visible
Complete the mechanism
Different geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. activated → different mature mRNAs → different proteins → specialised structure or function.
Avoid “different cells have different genes”
For typical nucleated somatic cells, explain differences in expression rather than a different inherited genome.
Key explanations
- Those proteins, such as contractile proteins or secreted enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction., give the cell its function.
- A gene can be present and still be switched off. Presence alone does not mean it is expressed.
Avoid this misconception
The claim: A specialised cell contains only the genes needed for its job.
The correction: Most specialised nucleated cells retain the genome; they express selected genes.
Order how a cell specialises
All the cells in a body have the same genes. They specialise because different genes are active. Order the chain.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put differential gene expression in order.
0 of 6 in the right place
Gene control is in 3C 3. Some differentiation is reversible in plants (totipotent cells), which the last step simplifies.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain why a muscle cell and a pancreatic beta cell can have different functions despite containing the same genes.
Show suggested answer
They express different subsets and amounts of genes, producing different mRNAs and proteins. Contractile proteins support muscle function; insulin and secretory proteins support beta-cell function.
Practice question
Retrieval and explanation · suggested answer
Does detecting gene X in a cell’s DNA show that protein X is being made?
Show suggested answer
No. The gene may not be transcribed; mRNA processing, translation and protein turnover also affect protein production.
Practice question
Retrieval and explanation · suggested answer
Why do two specialised cell types still express some of the same genes?
Show suggested answer
Both need shared cellular functions, such as basic metabolism and protein synthesis, alongside their specialised roles.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two cells contain equal amounts of a mature mRNA but different amounts of its protein. Has differential transcription necessarily caused the protein difference?
Show a hint
Consider events after transcription.
Explore the reasoning
No. Translation rates or protein breakdown can differ. Equal mRNA abundance also reflects both its production and degradation, so it is not a direct measure of transcription rate alone.
Recall this lesson
Lesson recall questions
Question 1 of 5: Explain, in causal order, why a palisade cell and a neurone can have the same genes but different structures.
Differential gene expression
Practice question
Retrieval and explanation · suggested answer
Explain, in causal order, why a palisade cell and a neurone can have the same genes but different structures.
Lesson: 3C 1: Cell Differentiation
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Reprogramming a specialised cell
3C · Cell DifferentiationThe work recognised by the 2012 Medicine Nobel Prize showed that mature differentiated cells can be reprogrammed to pluripotency. This challenged the idea that specialisation permanently removes the…Specification 3.18Read & use the scienceClose article
Why this matters
The work recognised by the 2012 Medicine Nobel Prize showed that mature differentiated cells can be reprogrammed to pluripotency. This challenged the idea that specialisation permanently removes the information needed for other cell types. Differentiation depends on which genes are transcribed into active mRNA and translated into proteins, not on each body cell having a separate genome. Reprogramming experiments make this point dramatically: changing gene activity can change what a mature cell is capable of becoming. Researchers must still test whether the resulting cells express the expected markers and behave safely. The Nobel discovery demonstrates developmental plasticity, but it does not mean that every ordinary differentiated cell spontaneously becomes pluripotent. In exam language, link gene expression to protein and then to cell structure or function.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.18 understand how cells become specialised through differential gene expression, producing active mRNA, leading to the synthesis of proteins which, in turn, control cell processes or determine cell structure in animals and plants
Use the science
Why does successful reprogramming support retention of genetic information?
Explore a possible response
The cell can regain a broad range of developmental possibilities, showing that specialised function did not require removal of all the information for other cell types.
Explore the source
- Nobel Prize 2012: cellular reprogrammingNobel Prize Outreach
The axolotl challenges a simple cell story
3C · Cell DifferentiationAxolotls can rebuild a lost limb, making them familiar in popular science. Researchers traced cell lineages during regeneration and found that cells retain information about their tissue of…Specification 3.18Read & use the scienceClose article
Why this matters
Axolotls can rebuild a lost limb, making them familiar in popular science. Researchers traced cell lineages during regeneration and found that cells retain information about their tissue of origin; the process is not simply every mature cell becoming a completely unrestricted stem cell. The paper is a reminder that differentiation and gene expression are more nuanced than a one-way arrow in a textbook diagram. It is an enrichment case, not a new species to learn for the exam. The syllabus explanation remains that active genes produce mRNA and proteins that shape specialised structure and function.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.18 understand how cells become specialised through differential gene expression, producing active mRNA, leading to the synthesis of proteins which, in turn, control cell processes or determine cell structure in animals and plants
Use the science
Why would limb regrowth alone not prove that every axolotl cell became totipotent?
Explore a possible response
Regrowth may arise from lineage-restricted progenitors and coordinated gene expression. Totipotency requires evidence of the full developmental potential, not just tissue repair.
Explore the source
- Cells keep a memory of tissue origin during axolotl regenerationNature · Abstract