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All cells in a multicellular organism contain the same DNA, yet they have very different structures and functions. How does a single fertilised egg give rise to the hundreds of different cell types in the human body? The answer lies in cell differentiation — the process by which cells become specialised. The Edexcel A-Level Biology specification (9BI0) requires an understanding of differentiation, stem cells and their potential applications.
Cell differentiation is the process by which a cell develops a more specialised structure and function by expressing certain genes while switching others off. During differentiation, the cell undergoes changes in:
All cells in an organism contain the same genome (the complete set of DNA). Differentiation occurs not by changes to the DNA sequence, but by differential gene expression — different genes are switched on (expressed) or off (silenced) in different cell types.
The mechanisms controlling gene expression include:
| Mechanism | Description |
|---|---|
| Transcription factors | Proteins that bind to specific DNA sequences (promoters/enhancers) to activate or repress transcription of target genes |
| Epigenetic modifications | Chemical modifications to DNA or histone proteins that alter gene expression without changing the DNA sequence. Examples include DNA methylation (adding methyl groups to cytosine bases, usually silencing gene expression) and histone acetylation (adding acetyl groups to histones, usually activating gene expression) |
| Cell signalling | Signals from neighbouring cells, hormones and growth factors influence which genes are expressed |
| mRNA processing | Alternative splicing of mRNA can produce different proteins from the same gene |
Exam Tip: A key concept to understand is that differentiation does not involve loss of genes. A differentiated cell still contains the entire genome — it simply expresses only a subset of genes appropriate to its function. This is proven by the success of cloning techniques such as somatic cell nuclear transfer (SCNT).
Stem cells are undifferentiated (or partially differentiated) cells that have two key properties:
| Type | Potency | Definition | Example |
|---|---|---|---|
| Totipotent | Highest | Can differentiate into any cell type in the organism, including extraembryonic tissues (e.g. placenta) | Zygote and cells up to the 8-cell stage of embryonic development |
| Pluripotent | High | Can differentiate into any cell type of the three germ layers (ectoderm, mesoderm, endoderm) but not extraembryonic tissues | Embryonic stem cells from the inner cell mass of the blastocyst |
| Multipotent | Moderate | Can differentiate into a limited range of cell types within a particular tissue or organ | Adult haematopoietic stem cells (produce all blood cell types); neural stem cells |
| Unipotent | Lowest | Can produce only one cell type (but can still self-renew) | Muscle satellite cells (produce only muscle cells); skin stem cells |
Embryonic stem cells (ESCs) are derived from the inner cell mass of a blastocyst — an early-stage embryo (approximately 5–7 days after fertilisation in humans). They are pluripotent and can be grown in culture to produce large numbers of cells.
The use of embryonic stem cells is ethically controversial because:
Adult stem cells (also called somatic stem cells) are found in various tissues throughout the body after development. They are generally multipotent — they can differentiate into a limited range of cell types within their tissue of origin.
| Location | Cell type | Differentiates into |
|---|---|---|
| Bone marrow | Haematopoietic stem cells | All types of blood cells (red blood cells, white blood cells, platelets) |
| Bone marrow | Mesenchymal stem cells | Bone, cartilage, fat, muscle cells |
| Brain | Neural stem cells | Neurons, astrocytes, oligodendrocytes |
| Skin | Epidermal stem cells | Keratinocytes and other skin cells |
| Gut lining | Intestinal stem cells | Epithelial cells of the intestinal villi |
In 2006, Shinya Yamanaka's team discovered that adult somatic cells (e.g. skin fibroblasts) could be reprogrammed to become pluripotent by introducing a small set of transcription factors (known as the Yamanaka factors: Oct4, Sox2, Klf4 and c-Myc). These reprogrammed cells are called induced pluripotent stem cells (iPSCs).
| Advantage | Explanation |
|---|---|
| No embryo destruction | Produced from adult somatic cells, avoiding the main ethical objection to embryonic stem cells |
| Patient-specific | Can be made from the patient's own cells, reducing the risk of immune rejection |
| Pluripotent | Have similar differentiation potential to embryonic stem cells |
| Drug testing | Can be used to create patient-specific cell models for testing drug responses |
Exam Tip: The Edexcel specification requires you to discuss the ethical and medical arguments surrounding the use of stem cells. Be prepared to present both sides of the debate in a balanced way. Always include specific examples and explain the science behind the ethical issues.
| Application | Description |
|---|---|
| Bone marrow transplant | Haematopoietic stem cells from a donor are transplanted into a patient with leukaemia or other blood disorders. The transplanted stem cells produce new, healthy blood cells |
| Skin grafts | Stem cells from a patient's own skin can be grown in culture and used to produce new skin for treating severe burns |
| Corneal repair | Limbal stem cells (from the edge of the cornea) can be used to regenerate damaged corneal tissue |
Plant cells retain a much greater capacity for differentiation than animal cells. Many differentiated plant cells can dedifferentiate — return to an undifferentiated state — and then redifferentiate into a different cell type. This property is called totipotency and is the basis of:
| Feature | Animal cells | Plant cells |
|---|---|---|
| Differentiation | Generally permanent and irreversible in most cells | Many cells retain the ability to dedifferentiate and redifferentiate |
| Totipotency | Only the zygote and very early embryonic cells | Many differentiated cells retain totipotency |
| Stem cell location | Specific niches (bone marrow, brain, gut, etc.) | Meristems (root tips, shoot tips, cambium) |
Exam Tip: Plant meristems are regions of undifferentiated cells that retain the ability to divide by mitosis and differentiate into any plant cell type. They are analogous to stem cell niches in animals. The root tip meristem is where you observe mitosis in the root tip squash practical.
The Edexcel 9BI0 specification places differentiation within Topic 2 (Cells, Viruses and Reproduction) with heavy overlap into Topic 3 (Voice of the Genome) because the molecular basis of differentiation is differential gene expression. Candidates must: define differentiation as the acquisition of specialised structure and function via regulated gene expression from an identical genome; classify stem cells by potency — totipotent (zygote and very early blastomeres; can form all cell types including extra-embryonic tissues such as trophoblast/placenta), pluripotent (inner cell mass of the blastocyst; all three germ layers but not extra-embryonic tissues), multipotent (tissue-specific adult stem cells such as haematopoietic stem cells in bone marrow giving all blood lineages) and unipotent (e.g. muscle satellite cells); explain how transcription factors binding promoters/enhancers and chromatin modifications (DNA methylation, histone acetylation) drive cell-type-specific gene expression; evaluate ethical and medical trade-offs of embryonic stem cells (ESCs) versus induced pluripotent stem cells (iPSCs) versus adult stem cells; and discuss therapeutic applications (Parkinson's, type-1 diabetes, spinal-cord injury, leukaemia). Synoptic links: lesson 9 (gamete fusion produces the totipotent zygote), Topic 3 (transcription factors, epigenetics and the genome's "voice"), Topic 6 (cancer as failure of differentiation control plus tumour-suppressor loss), Topic 8 (regenerative medicine, gene therapy applications), Topic 5 (plant meristems as the equivalent stem-cell pool) — refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
A patient with Parkinson's disease has progressive loss of dopaminergic neurons in the substantia nigra. A researcher proposes treating the patient with stem-cell-derived dopaminergic neurons.
(a) A neuron and a hepatocyte from the same patient contain identical genomes yet have completely different structures and functions. Explain how this is possible. (3)
(b) Compare the suitability of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) as the source for this therapy, considering both science and ethics. (3)
(c) Classify each of the following by potency, justifying each answer: a zygote; a cell from the inner cell mass of a blastocyst; a haematopoietic stem cell; a muscle satellite cell. (2)
Solution with mark scheme:
(a) M1 (AO1.1) — both cells were derived from the same zygote by mitosis, which produces genetically identical daughters; so neuron and hepatocyte contain the same DNA sequence (i.e. the same genome).
A1 (AO1.2) — the cells differ because of differential gene expression: different subsets of genes are switched on (transcribed and translated) in each cell type while others are silenced. A neuron expresses voltage-gated ion channels and neurotransmitter-receptor genes; a hepatocyte expresses albumin, cytochrome P450 and urea-cycle enzymes.
A1 (AO2.1) — control of gene expression is mediated by transcription factors binding cell-type-specific promoters/enhancers, and by chromatin modifications (e.g. DNA methylation of cytosines silences genes; histone acetylation opens chromatin and activates genes). Differentiation is therefore an epigenetic outcome from an unchanged genome — not a loss of genes.
(b) M1 (AO1.2) — scientifically both ESCs and iPSCs are pluripotent and can be directed in vitro to dopaminergic neuron lineages; ESCs are the historical gold standard, iPSCs derived from the patient's own fibroblasts give an autologous match minimising immune rejection.
M1 (AO2.1) — ethically, ESC isolation requires destruction of a blastocyst-stage embryo, which is contested; iPSC reprogramming uses adult somatic cells and does not destroy an embryo, removing the principal ethical objection.
A1 (AO3.2a) — outstanding caveat: iPSCs may retain residual epigenetic memory of the donor somatic cell, reprogramming efficiency is low, and some Yamanaka factors (notably c-Myc) are proto-oncogenes, raising tumour-formation risk that ongoing clinical trials are still characterising. A balanced answer cites both the ethical advantage and these unresolved safety questions.
(c) M1 (AO1.1) — zygote = totipotent (can form all cell types including extra-embryonic trophoblast/placenta); inner cell mass cell = pluripotent (three germ layers but not extra-embryonic tissues).
A1 (AO1.2) — haematopoietic stem cell = multipotent (all blood lineages — erythrocytes, leukocytes, platelets — but not, e.g., neurons); muscle satellite cell = unipotent (gives only skeletal-muscle cells, though it self-renews).
Total: 8 marks.
Question (6 marks): Cells with identical genomes can have radically different structures and functions. Discuss the molecular mechanisms by which differentiation is achieved, and evaluate the therapeutic potential of stem cells for treating degenerative disease.
Mark scheme decomposition by AO:
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