What you need to know
- 3.17Stem-cell definitions and societal decisions.
- 3.18–3.19Differential expression and post-transcriptional processing.
- 3.20–3.21epigeneticDescribes changes in gene expression that do not change the DNA base sequence, such as DNA methylation or histone modification., genotypeThe alleles an organism has for a gene.–environment interaction and variation.
By the end, you should be able to…
- Explain phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment. from geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. activity to protein function.
- Distinguish related genetic concepts precisely.
- Evaluate evidence and ethical arguments.
Build your understanding
Build the explanation in order
For differentiationThe process by which a cell becomes specialised, because some genes are switched on and others are switched off., start with geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. activation, then mature messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome., protein production and the resulting cell structure or function. For alternative splicing, begin with one transcript and show how different retained exons can produce different coding messages.
For epigeneticDescribes changes in gene expression that do not change the DNA base sequence, such as DNA methylation or histone modification., identify the mark, its possible effect on chromatin or regulatory binding, and the consequence for transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus.. Do not replace the explanation with the phrase “the gene changes” when its DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. sequence has not changed.
Compare like with like
multiple allelesWhen a gene has more than two alleles in a population, such as the ABO blood group gene. concerns alternatives at one locusThe position of a gene on a chromosome.; polygenicDescribes a characteristic controlled by many genes, often giving continuous variation. inheritance concerns several loci. Totipotency and pluripotency describe different developmental ranges. Morula and blastocyst describe embryo structures, not interchangeable classes of stem cellAn undifferentiated cell that can divide and differentiate into specialised cell types..
In evidence questions, distinguish a measured association, a proposed mechanism and a demonstrated causal effect. In ethical discussions, make the scientific basis clear while recognising where values influence the conclusion. All questions and suggested answers below are original, without official mark allocations.
Make your reasoning visible
Use a specific noun
State whether DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. sequence, methylation, RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. sequence or protein amount differs.
Qualify the conclusion
A laboratory observation may support a mechanism without establishing a safe therapy.
Link every comparison to the question
A correct definition is useful only if you apply it to the given cells, traits or evidence.
Avoid this misconception
The claim: One geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. always gives one protein and one phenotypeThe observable characteristics of an organism, produced by its genotype interacting with the environment..
The correction: RNA processing can produce several protein variants, and phenotypes can involve many genes and environmental effects.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Explain how two nucleated somatic cell types can retain the same DNA but produce different proteins.
Show suggested answer
They activate different genes or express them at different levels, generating different mature mRNA profiles. Translation produces different protein sets or amounts, affecting structure and function.
Practice question
Retrieval and explanation · suggested answer
A pre-mRNA contains exons A–B–C–D. Two valid coding messages contain A–B–D and A–C–D. Explain how they arise and a possible consequence.
Show suggested answer
Alternative splicing retains different exon combinations after transcription. Different coding sequences can produce polypeptides with different amino acid sequences and therefore different properties.
Practice question
Retrieval and explanation · suggested answer
Distinguish promoter methylation from mutation.
Show suggested answer
Methylation adds chemical marks without changing DNA base sequence and can alter transcription. Mutation changes the sequence itself.
Practice question
Retrieval and explanation · suggested answer
Explain why daughters of a specialised cell can retain its expression pattern.
Show suggested answer
Maintenance of epigenetic marks and regulatory states after replication and division can preserve similar gene activity, leading to continued production of specialised proteins.
Practice question
Retrieval and explanation · suggested answer
Explain how polygenic inheritance and environment can yield continuous variation.
Show suggested answer
Alleles at several loci produce many combinations of contributions. Environmental influences add further variation, creating a range of phenotypic values rather than only a few distinct categories.
Practice question
Retrieval and explanation · suggested answer
A cell forms many body cell types but cannot form the full set of extraembryonic lineages. Which potency term fits, and why?
Show suggested answer
Pluripotent: broad body-cell potential without the complete developmental potential required for totipotency.
Practice question
Retrieval and explanation · suggested answer
A proposal uses embryo-derived stem cells to make replacement tissue. Give two biological requirements and two ethical considerations.
Show suggested answer
Biological requirements include correct functional differentiation and controlled growth or integration. Ethical considerations include informed consent, moral status of the embryo, fair access or balancing expected benefits against uncertain harms; explain their relevance rather than merely listing them.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A treatment changes mRNA abundance, a histone mark and cell shape. Put these observations into a possible causal chain, then state why the chain is not proven.
Show a hint
A sequence of plausible events still needs a test of direction.
Explore the reasoning
One hypothesis is altered histone state → altered transcription → changed protein production → altered shape. The observations could have other causes or occur in a different order; controlled perturbations and time-resolved evidence are needed.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare totipotent and pluripotent stem cells.
Stem cells and therapies
Practice question
Retrieval and explanation · suggested answer
Compare totipotent and pluripotent stem cells.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Looking beyond a gene count
3C · Exam PracticeNHGRI explains that alternative splicing allows different exon combinations to form related mRNAs. Counting genes therefore does not tell us exactly how many protein variants a cell can…Specification 3.19Read & use the scienceClose article
Why this matters
NHGRI explains that alternative splicing allows different exon combinations to form related mRNAs. Counting genes therefore does not tell us exactly how many protein variants a cell can produce. After transcription, an RNA transcript can be processed so that different exon combinations remain. One gene can therefore give rise to more than one mRNA and potentially more than one protein. This is a mechanistic explanation, not a claim that a cell makes every possible variant. In an exam answer, distinguish DNA sequence, pre-mRNA, mature mRNA and translated product. Researchers can sequence RNA to discover variants, then measure proteins to test whether those messages actually yield a functional molecule.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.19 understand how one gene can give rise to more than one protein through post-transcriptional changes to messenger RNA (mRNA)
Use the science
Why does the number of possible RNA variants not equal the number of functional proteins observed?
Explore a possible response
Some messages may not be translated or may be degraded; protein stability and function also differ. A possible product is not the same as a demonstrated functional product.
Explore the source
- NHGRI: Alternative SplicingNational Human Genome Research Institute
A medicine that changes RNA processing
3C · Controlling Gene ExpressionThe US Food and Drug Administration describes risdiplam as an RNA-splicing modifier for spinal muscular atrophy. Its action illustrates why processing a transcript matters: a change after transcription…Specification 3.19Read & use the scienceClose article
Why this matters
The US Food and Drug Administration describes risdiplam as an RNA-splicing modifier for spinal muscular atrophy. Its action illustrates why processing a transcript matters: a change after transcription can alter the protein ultimately produced without rewriting the patient’s DNA sequence. The disease and treatment are enrichment, not named facts required by the specification. To connect the example back to Unit 2, trace the sequence from a gene to pre-mRNA, processed mRNA and protein. Avoid calling an RNA-processing medicine a gene-editing treatment; those mechanisms answer different questions.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.19 understand how one gene can give rise to more than one protein through post-transcriptional changes to messenger RNA (mRNA)
Use the science
Why is changing mRNA splicing different from changing the DNA base sequence?
Explore a possible response
Splicing acts on the RNA transcript after transcription. The DNA sequence remains, while the processed message and resulting protein can differ.
Explore the source
- FDA approves oral treatment for spinal muscular atrophyUS Food and Drug Administration
A stem-cell claim needs a safety question
3C · Stem CellsAdvertisements sometimes treat the words stem cell as proof of a cure. The FDA warns that many regenerative-medicine products marketed for a wide range of conditions have not…Specification 3.17(i) · 3.17(ii)Read & use the scienceClose article
Why this matters
Advertisements sometimes treat the words stem cell as proof of a cure. The FDA warns that many regenerative-medicine products marketed for a wide range of conditions have not been shown to be safe and effective. A cell’s ability to divide and differentiate is biologically interesting, but it does not establish where it will go after treatment or whether it will grow in the right way. This is a direct application of stem-cell potency and societal decision-making in the specification. Students can ask what cell type was used, what outcome was measured, and whether a controlled trial supports the claim.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.17(i) understand what is meant by the terms stem cell, pluripotent and totipotent, morula and blastocyst
- 3.17(ii) be able to discuss the ways in which society uses scientific knowledge to make decisions about the use of stem cells in medical therapies
Use the science
Why does a pluripotent cell’s broad potential not by itself prove that an injection will repair a damaged joint?
Explore a possible response
Potential in suitable conditions is not controlled differentiation at the target site. Safety, delivery, function and clinical benefit all need evidence.
Explore the source
- Important patient information about regenerative medicineUS Food and Drug Administration