Explain how epigenetic modifications to histone proteins can result in the silencing of gene expression without altering the nucleotide sequence of DNA.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Histones are the proteins around which DNA is wrapped to form nucleosomes. Chemical changes to the histone tails – for example, methylation of lysine residues or deacetylation of lysine side chains – make the chromatin fibre more compact, forming heterochromatin. In this condensed state the DNA is inaccessible to transcription factors and RNA polymerase, so transcription cannot be initiated. Because the DNA sequence itself is unchanged, the effect is epigenetic and can be inherited or reversed.
Examiner tips
- Use the word ‘histones’ and ‘nucleosomes’ early. Mention specific modifications (methylation, deacetylation) and the result (heterochromatin). Explain the consequence for transcription. State that the DNA sequence is unchanged and the change is epigenetic.
Common mistakes
- Confusing DNA methylation with histone modification. Failing to mention that chromatin becomes inaccessible to transcription machinery. Using vague terms like ‘changes’ without specifying methylation or deacetylation.
Mark scheme (4 marks)
- Histones are proteins around which DNA is wound/wrapped to form nucleosomes
- Chemical modifications such as methylation of histone tails (or deacetylation of lysine residues) cause chromatin to condense/become more tightly packed (heterochromatin)
- Condensed chromatin is inaccessible to transcription factors and/or RNA polymerase, so transcription cannot be initiated
- Because the DNA nucleotide sequence itself is unchanged, the modification is described as epigenetic and can be heritable/reversible
Key terms in this question
epigenetic modification · histone
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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