3.8.2.2Control of Gene ExpressionCore

Epigenetics and Environmental Influences

Cells can maintain differences in gene activity without changing the underlying base sequence. Explain the molecular mark and its regulatory consequence rather than calling every environmental effect a mutation.

35 min Regulation, Differentiation and Cancer

What you'll be able to do

Explain DNA methylation and histone acetylation
Distinguish epigenetic regulation from sequence mutation
Evaluate inheritance and disease-related evidence

Relate chromatin state to gene accessibility

Eukaryotic DNA is associated with histones and other proteins. The organisation of this chromatin affects access by transcriptional machinery. In the syllabus model, reduced acetylation of associated histones can favour a less accessible state and reduce transcription.

DNA methylation adds chemical groups without replacing the base sequence. Increased methylation at appropriate regulatory regions can inhibit transcription. The effect depends on location and context, so a methylation measurement somewhere in a genome is not a universal instruction that every gene is switched off.

Distinguish regulatory memory from a new allele

Epigenetics concerns changes in gene activity or function maintained through cellular inheritance without a corresponding base-sequence change. Regulatory marks can help preserve a differentiated cell’s expression pattern as it divides. Environmental conditions can influence some of these marks and the associated expression.

Inheritance through a cell lineage is not identical to guaranteed transmission through sexual generations. Many marks are reset during reproduction and development, and evidence for an intergenerational effect must address alternative genetic and environmental explanations. Avoid claiming that every acquired experience becomes permanently encoded for all descendants.

Evaluate causal claims and disease relevance

If a gene is methylated and expressed less, the observations show an association. Manipulating the mark, comparing suitable controls and measuring transcription can help test whether the change contributes causally. Differences in cell types within a sample can also affect the measured average.

Abnormal regulation can contribute to disease, including altered expression of genes controlling cell division. A proposed treatment that changes epigenetic regulation requires evidence about target genes, specificity, function and consequences elsewhere. A reversible mark does not guarantee that all resulting effects are immediately reversible or harmless.

Apply the biology

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.

Example 1Two cell types have the same coding sequence for a gene, but one has more promoter-region methylation and less mRNA. Suggest a mechanism and one limitation.

1Methylation at that regulatory region could reduce transcriptional access or activity.

2The association alone does not prove causation or exclude other regulatory differences.

AnswerA plausible epigenetic mechanism needs a discriminating test.

Example 2Why is a chemically methylated DNA region not necessarily a new allele in the sequence-based sense used here?

1The underlying order of bases can remain unchanged.

2The modification affects regulation rather than replacing the sequence.

3A mutation would involve a change in that base sequence.

AnswerDistinguish a regulatory modification from a sequence variant.

Common mistakes to avoid

Calling all epigenetic regulation a change in base sequence.

Identify the modification and expression effect.

Extending cell-lineage inheritance automatically to every future generation.

Check reproductive resetting and the evidence.

Key takeaways

Chromatin and DNA modifications can regulate access to genes.
Expression memory differs from sequence mutation.
Environmental and disease claims need causal and inheritance evidence.

Test yourself

Answer a few questions, then review anything you missed.

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