What you need to know
- 3.18geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. expression produces messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. and proteins.
- 3.19Post-transcriptional mRNA changes can produce more than one protein from one gene.
- 3.20(ii–iii)DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. methylation, histone modification and inheritance through cell division.
By the end, you should be able to…
- Explain alternative splicing.
- Link chromatin changes to transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus..
- Distinguish epigeneticDescribes changes in gene expression that do not change the DNA base sequence, such as DNA methylation or histone modification. inheritance through mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent. from changes to DNA sequence.
Build your understanding
Controlling access to a gene
transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus. depends on regulatory molecules interacting with DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. and on access to the DNA. In eukaryotes DNA is associated with histone proteins in chromatin. Changes in chromatin organisation can make a region more or less accessible to transcription machinery.
A geneA sequence of DNA bases that codes for a polypeptide or a functional RNA. may be present but transcribed at a low rate or not at all. Regulation is often a matter of how much product is made, rather than a permanently on or off switch.
DNA methylation and histone modification
DNA methylation adds methyl groups to DNA without changing its base sequence. Increased methylation in a gene’s promoter region is commonly associated with reduced transcription, for example by affecting regulatory-protein binding and recruiting proteins that compact chromatin. It is not correct to claim that methylation at every position always switches every gene off.
Chemical modifications to histones can alter chromatin accessibility or recruit regulatory proteins. Histone acetylation is commonly associated with more accessible chromatin and increased transcription; deacetylation often has the opposite association. Different histone methylation marks can have different effects. Explain the consequence for transcription rather than treating all modifications as identical.
Maintaining a cell identity
After DNA replication, maintenance mechanisms can restore methylation patterns on newly synthesised DNA. Histone-associated states can also help re-establish patterns of gene activity after division. Such epigeneticDescribes changes in gene expression that do not change the DNA base sequence, such as DNA methylation or histone modification. memory helps daughter cells maintain specialised characteristics.
Passing an epigenetic state through mitosisNuclear division that produces two genetically identical nuclei with the same number of chromosomes as the parent. does not mean every environmental effect is inherited by future generations. Transmission through gameteA haploid sex cell, such as a sperm or an egg cell. is a separate question, and many epigenetic marks are reset during reproduction and development. A stable mark is also not automatically an irreversible mark.
One gene, different mature messages
An initial RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. transcript can contain exons and introns. During RNA processing introns are removed and retained exons are joined. Alternative splicing joins different combinations of exons, producing different mature mRNAs from the same gene.
If those differences alter the translated coding sequence, different polypeptides can be produced. Their amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. sequences affect folding and function. In an illustrative model, a transcript with exons 1–2–3–4 gives mature mRNAs 1–2–4 and 1–3–4. Assume both are valid coding messages: they can encode distinct protein variants.
This processing changes RNA after transcription; it does not rearrange the cell’s DNA. Not every splice variant necessarily produces a stable functional protein, so use the assumptions supplied in a question.
NOT: methylation is not a mutation, and an intron is not kept in the mature mRNA.
- One gene, more than one protein, is alternative splicing
- Epigenetic marks can pass through mitosis
- That is not the same as changing the DNA bases
- A mutation would change the sequence itself
Two traps this diagram blocks
- Do not say every exon must appear in every mRNA from that gene.
- Do not call methylation a substitution mutation.
From the diagram alone
Question
Explain how one gene can code for two polypeptides without a mutation. [3]
Show a model answer
The pre-mRNA can be spliced in more than one way. Introns are removed, and a different combination of exons is kept in each mature mRNA. Translation of those mRNA molecules produces two polypeptides. The DNA sequence need not change.
Make your reasoning visible
Locate the change
Mutation changes DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. sequence; methylation changes chemical marks; alternative splicing changes RNARibonucleic acid: a single-stranded polynucleotide with ribose and uracil. mRNA, tRNA and rRNA take part in protein synthesis. processing.
Complete the expression link
Altered chromatin accessibility → altered transcriptionMaking an mRNA copy of a gene, using one strand of DNA as a template, in the nucleus. → altered messenger RNAA single-stranded RNA copy of a gene that carries the code from the nucleus to a ribosome. → altered protein production.
State the inheritance route
Specification 3.20(iii) concerns maintenance after cell division. Do not assume universal inheritance from parents to children.
Key explanations
- Splicing removes introns. The remaining exons are joined.
Avoid this misconception
The claim: Alternative splicing deletes unwanted exons from the DNA.
The correction: It changes which exons are retained in a mature RNA; the geneA sequence of DNA bases that codes for a polypeptide or a functional RNA.’s DNA remains present.
Same genes, different cells?
A nerve cell and a skin cell carry the same genes, yet they make different proteins. How does a cell decide which genes to use, and how much?
What the model shows
A gene in its chromatin, the marks and proteins that control it, splicing of its mRNA, and how much mRNA and protein the cell makes.
What to look for
Which changes lower transcription, which change the protein made, and what happens to the pattern when the cell divides.
What it tells us
Genes can be switched up or down without changing the DNA sequence. One gene can make more than one protein.
Be careful what you claim
Illustrative relative amounts; each mark is simply on or off.
Try it in the model: set Transcription factor to Absent and watch the graph.
Switching a gene up and down
Control one gene: add or remove a transcription factor, methylate its promoter, change histone acetylation, divide the cell and choose how its RNA is spliced. Amounts are illustrative relative units.
What you see
- A stretch of chromatin: a promoter and a gene with four exons, wound around histones. Methyl groups (CH₃) can sit on the promoter DNA, and acetyl groups (Ac) on the histones. A transcription factor and RNA polymerase bind when they can.
- Below, the pre-mRNA is spliced into mature mRNA and translated. The graph shows the amount of mRNA over time, and the meters show the transcription rate and the protein.
How to use it
- Add or remove the transcription factor, methylate the promoter, change histone acetylation, divide the cell, and choose how the pre-mRNA is spliced.
What it shows
- Gene expression can be controlled by transcription factors and by chromatin: promoter methylation and histone deacetylation are commonly associated with less transcription, acetylation with more (3.18, 3.20(ii)).
- Epigenetic marks can be copied when a cell divides, keeping the pattern of gene activity (3.20(iii)). Alternative splicing makes different mRNAs, and so different proteins, from one gene (3.19).
Limits of the model
- Rates and amounts are illustrative relative units. Each mark is reduced to on or off, and the effects are simple multipliers. In real cells the effect of a mark depends on where it is, and different histone marks have different effects.
- Marks copied through mitosis are not automatically passed to children: many are reset in gametes and early development. The model does not show that.
Guided tour · step 1 of 8
- In Controls, just below these steps, set Transcription factor to Absent.
- Watch the mRNA graph.
Without the transcription factor bound to the promoter, RNA polymerase is rarely recruited. Transcription drops to a low basal rate (5% here), so mRNA and then protein fall. The gene is still there; it is just hardly used.
Controls
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
How can one gene produce two proteins without mutation?
Show suggested answer
Its transcript can undergo alternative splicing to form mature mRNAs with different exon combinations. If their coding sequences differ, translation can produce different polypeptides.
Practice question
Retrieval and explanation · suggested answer
Explain how increased promoter methylation could reduce protein production.
Show suggested answer
It can reduce transcriptional access or regulatory binding, lowering transcription and mature mRNA availability; less mRNA can reduce translation and protein production.
Practice question
Retrieval and explanation · suggested answer
Why does epigenetic memory help a specialised tissue persist?
Show suggested answer
Daughter cells can maintain similar patterns of gene activity after division, continuing production of proteins associated with the tissue’s function.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A promoter is highly methylated and its gene has little mRNA. Does the association prove methylation caused the low mRNA?
Show a hint
Consider an intervention and alternative causes.
Explore the reasoning
No. Other regulators or RNA degradation may contribute. A controlled alteration of methylation with measurements of transcription and appropriate controls would strengthen a causal explanation.
Recall this lesson
Lesson recall questions
Question 1 of 5: List the three linked steps the spec gives from genes to a specialised animal or plant cell.
Differential gene expression
Practice question
Retrieval and explanation · suggested answer
List the three linked steps the spec gives from genes to a specialised animal or plant cell.
Lesson: 3C 1: Cell Differentiation
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Mapping gene regulation
3C · Controlling Gene ExpressionNHGRI describes epigenomics as the study of chemical marks associated with the genome. Comparing cell types can reveal patterns associated with different gene activity. An epigenetic mark is…Specification 3.20(ii)Read & use the scienceClose article
Why this matters
NHGRI describes epigenomics as the study of chemical marks associated with the genome. Comparing cell types can reveal patterns associated with different gene activity. An epigenetic mark is a chemical modification associated with DNA or histones, not a change to the base sequence. Comparing marks between cell types can suggest how genes are activated or silenced. Researchers also measure mRNA and protein because a mark by itself is not proof of a particular functional outcome. Some marks persist when cells divide, helping maintain specialised states. That does not mean every life experience is inherited by children. The exam chain is mark, altered gene activation, changed mRNA or protein, and possible phenotype.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.20(ii) know how epigenetic modification, including DNA methylation and histone modification, can alter the activation of certain genes
Use the science
Why compare DNA sequence as well as epigenetic marks?
Explore a possible response
A sequence difference could provide another explanation for altered expression; comparing both helps separate the possibilities.
Explore the source
- NHGRI: Epigenomics Fact SheetNational 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