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3.4.4Topic 3 · Voice of the Genome

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

5 min read 9 parts Lesson 23 of 26 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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3.4.4 Epigenetics — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how DNA methylation and histone modification control gene expression without changing the base sequence, and discuss their consequences.Epigenetics is the study of heritable changes in gene expression that a…

  • Acetylation
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Nucleosome
  • Euchromatin

Section 1 of 9What is epigenetics?

Epigenetics is the study of heritable changes in gene expression that are not caused by changes to the DNA base sequence.

What is epigenetics?
Epigenetic marks are chemical tags added to DNA or to the proteins around it.
They act like switches that turn genes on or off,
  • and they can be influenced by the environment,
  • such as diet, stress, smoking or exposure to chemicals.
Some marks are passed on when cells divide, so that a liver cell divides to give more liver cells with the same pattern of expressed genes.

Section 2 of 9DNA methylation

DNA methylation is the addition of a methyl group (CH3) to cytosine bases, usually where a cytosine is followed by a guanine (CpG sites), in promoter regions of genes.

DNA methylation
An enzyme, DNA methyltransferase, adds the methyl groups.
Increased methylation of a promoter generally makes the gene less likely to be transcribed,
  • because methylated DNA is not recognised by transcription factors and attracts proteins that make the chromatin more compact.
Decreased methylation (hypomethylation) allows the gene to be switched on.

Methylation patterns are copied during DNA replication by maintenance methyltransferases, so the pattern is passed to daughter cells.

Section 3 of 9Histone modification

DNA is wound around histone proteins to form nucleosomes. The state of this packaging affects whether genes are accessible.

Acetylation
(adding acetyl groups to histone tails) reduces the positive charge on histones, so they bind DNA less tightly.
  • The chromatin is open (euchromatin), allowing transcription factors and RNA polymerase to reach the gene, so transcription increases.
Deacetylation
Deacetylation and the addition of methyl groups to histones can make the DNA bind more tightly.
  • The chromatin is condensed and genes are not accessible, so transcription is reduced.
    • (heterochromatin)
ModificationEffect on chromatinEffect on gene expression
DNA methylationMore condensedGene silenced
Histone acetylationLooserGene activated
Histone deacetylationTighterGene silenced

Section 4 of 9Consequences and evidence

Epigenetic change explains why identical twins, who share the same DNA, become less alike as they age: they build up different epigenetic marks according to differences in lifestyle and environment.

1
Another example is X-chromosome inactivation in female mammals, where one X in each cell is condensed and silenced.
2
Epigenetic changes are also implicated in cancer: methylation silences tumour suppressor genes, while loss of methylation can switch on oncogenes.
3
Drugs that inhibit DNA methylation or histone deacetylases are used to treat some cancers, by reactivating silenced tumour suppressor genes.
4
Studies suggest that some epigenetic marks can be inherited by the next generation, for example that nutrition of grandparents is associated with disease risk in grandchildren,
  • but this area is still under investigation.

Section 5 of 9Epigenetics and the environment

Epigenetics shows how nature and nurture interact.

1
A genotype provides a range of possible phenotypes, and the environment can influence which genes are expressed within that range.
2
Epigenetic effects can be reversed, unlike mutations, which are permanent changes in the sequence.
3
Because of this, they offer possible targets for treatment.
4
They also help to explain why a cell keeps its specialised identity: the pattern of marks is a form of cellular memory.

Section 6 of 9Exam focus and links

Exam questions on epigenetics frequently provide data such as the percentage of methylation in a gene promoter in two groups, together with the amount of mRNA or protein produced.

Exam focus and links
Describe the relationship: as methylation increases, expression decreases.
State the type of correlation and then explain the mechanism using the terms you have learned.
Be careful to say that the DNA sequence is the same in both groups.

Remember that epigenetics helps to explain how the same genome can give different cell types, how the environment can alter risk of disease, and why cancer treatments aim at epigenetic enzymes.

Exam focus and links
It also links to the idea that a phenotype is the result of interaction between genotype and environment, which is the next lesson.
Use correct vocabulary: histone tails are modified, DNA is methylated, and chromatin is condensed or relaxed.
Do not say that genes are 'deleted'.

One more example worth knowing is the effect of early-life nutrition: studies of people exposed to famine before birth found altered methylation of some growth-related genes and a higher later risk of metabolic disease. Such correlations suggest that the environment can leave a lasting epigenetic imprint, although proving causation in humans is difficult and controlled experiments are mostly done in animals.

Section 7 of 9Exam-style questions

  1. Explain how increased methylation of a gene's promoter can lead to reduced production of a protein. [3]3 marks

  2. Describe how histone acetylation affects gene expression. [3]3 marks

  3. Suggest why identical twins show increasing differences in disease risk as they age. [2]2 marks

Section 8 of 9Key terms

TermMeaning
EpigeneticsHeritable changes in gene expression not caused by changes in DNA sequence.
DNA methylationAddition of methyl groups to cytosine in DNA, usually silencing genes.
Histone acetylationAddition of acetyl groups to histones, loosening chromatin and activating genes.
NucleosomeDNA wrapped around a core of histone proteins.
EuchromatinLoosely packed chromatin in which genes can be expressed.
HeterochromatinTightly packed chromatin in which genes are silenced.
Tumour suppressor geneA gene whose protein slows cell division; loss of function can lead to cancer.

Section 9 of 9Exam technique

1
Saying that methylation changes the DNA base sequence. It adds a chemical group but the sequence is unchanged.
2
Writing that acetylation silences genes. Acetylation generally activates, while deacetylation silences.
3
Thinking all epigenetic changes are permanent. They can often be reversed.
4
Confusing methylation of cytosine in DNA with the methylation of histones.

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