3.4.3Topic 3 · Voice of the Genome

Cell Differentiation — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision

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3.4.3 Cell Differentiation — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how differential gene expression produces specialised cells and describe the roles of transcription factors and cell signalling.Differentiation is the process by which an unspecialised cell becomes …

Exam-style question 1
Exam-style question 2
Exam-style question 3
Housekeeping gene
Almost all the body cells of an organism contain the same genome, because they came from the zygote by mitosis
The cells differ because they express different genes, so each makes a different set of proteins

Section 1 of 9Differentiation and the genome

Differentiation is the process by which an unspecialised cell becomes a specialised one with a particular structure and function.

Almost all the body cells of an organism contain the same genome, because they came from the zygote by mitosis.

The cells differ because they express different genes, so each makes a different set of proteins.

An insulin gene is active in a pancreatic beta cell, for instance, but not in a red blood cell precursor.

4Differential gene expression

This is differential gene expression.

Evidence that specialised cells retain the whole genome comes from cloning: a nucleus taken from a differentiated cell of an adult sheep, transferred into an enucleated egg, produced Dolly the sheep, showing that the nucleus still contained all the genes needed for development.

Section 2 of 9Regulation of transcription

Control of gene expression is mainly through whether a gene is transcribed. A gene is switched on when RNA polymerase is able to bind to its promoter and begin transcription. This is influenced by transcription factors, proteins that bind to specific DNA sequences near the gene.

Regulation of transcription

An activator transcription factor binds to a regulatory region and helps RNA polymerase to bind, so increasing transcription.

A repressor binds and blocks transcription.

Many transcription factors need to be activated first, for example by binding a hormone such as oestrogen,

which enters the cell, binds to a receptor and the complex acts as a transcription factor.

The lac operon of E. coli is a classic model: when lactose is present it binds the repressor protein, so the repressor can no longer block the operator, and RNA polymerase transcribes the genes for lactose digestion.

Section 3 of 9Cell signalling during development

Cells in an embryo are influenced by chemical signals from neighbouring cells and by gradients of signalling molecules (morphogens).

Cells at different distances from the source receive different concentrations and so switch on different sets of genes.

This determines position and fate in the body plan.

Homeobox (Hox) genes code for transcription factors that determine the body plan along the head-to-tail axis, and are highly conserved among animals.

Once differentiation has occurred, the pattern of expressed genes is usually stable and is passed on when the cell divides, which is where epigenetic modification becomes important. Some cells, such as skin and blood cells, keep a stem cell population so that tissue can be replaced.

Section 4 of 9Examples of specialised cells

Cell	Genes expressed	Resulting specialisation
Erythrocyte	Haemoglobin genes; nucleus lost	Oxygen transport
Beta cell of pancreas	Insulin gene	Secretes insulin
Muscle fibre	Actin and myosin genes	Contraction
Xylem vessel	Lignin synthesis genes	Support and water transport

Some genes, called housekeeping genes, are expressed in all cells because they code for basic functions such as respiratory enzymes and ribosomal proteins.

Section 5 of 9Differentiation in plants and cancer

In plants, cells produced in the meristem become xylem, phloem, epidermis or mesophyll depending on position and hormones such as auxin.

Differentiation in plants and cancer

Cell differentiation in plants can be reversed when cells are cultured in the appropriate hormones, a feature used in micropropagation.

In cancer, cells lose their specialised properties and divide uncontrollably, because mutations affect genes that regulate the cell cycle.

The idea that differentiation is controlled by gene expression is therefore central to developmental biology and to understanding disease.

Section 6 of 9Exam focus and links

For questions about gene expression, you should always describe the whole chain: signal or factor, binding to DNA, effect on RNA polymerase, transcription, translation and the resulting protein that gives the cell its feature.

Exam focus and links

Link this to protein synthesis from earlier in the course, where you studied transcription and translation.

Remember that the mRNA is a copy of a gene,

so cells that make a lot of one protein have a lot of the corresponding mRNA,
- and that some regulation also occurs after transcription,
- such as the editing of RNA and the control of translation.

Link to stem cells

as differentiation proceeds, more genes are permanently silenced, which is why potency falls.
In cancer and in reprogramming to make iPS cells, the pattern of expression is reversed or disrupted by changing which transcription factors are active.
In plants the hormone auxin influences gene expression and cell elongation in a similar way.

Section 7 of 9Exam-style questions

Explain how two cells in the same organism can have identical DNA but different structures and functions. [3]3 marks
Describe how a transcription factor can switch on a gene. [3]3 marks
Explain what the cloning of Dolly the sheep showed about differentiated cells. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Differentiation	The process by which a cell becomes specialised.
Differential gene expression	Different cells switching on different genes.
Transcription factor	A protein that binds to DNA to increase or decrease transcription of a gene.
Promoter	DNA sequence where RNA polymerase binds to begin transcription.
Morphogen	A signalling molecule whose concentration gradient determines cell fate.
Housekeeping gene	A gene expressed in all cells for basic functions.

Section 9 of 9Exam technique

Saying that differentiated cells lose the genes they do not use. The genes are still present but not expressed.

Believing that a hormone like oestrogen binds to the DNA directly. It binds to a receptor and the complex is the transcription factor.

Confusing transcription factor with enzyme

transcription factors are regulatory proteins;
- RNA polymerase is the enzyme.

Forgetting that mature red blood cells and sieve tubes lose their nuclei during differentiation.

Last updated: 18 October 2026