# Explain how epigenetic modifications to histone proteins can result in the silencing of gene expression without altering the nucleotide sequence of DNA.

> IB DP Biology Higher Level (2023 syllabus) — D2.2 Gene expression (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. Histones are proteins around which DNA is wound/wrapped to form nucleosomes
2. Chemical modifications such as methylation of histone tails (or deacetylation of lysine residues) cause chromatin to condense/become more tightly packed (heterochromatin)
3. Condensed chromatin is inaccessible to transcription factors and/or RNA polymerase, so transcription cannot be initiated
4. Because the DNA nucleotide sequence itself is unchanged, the modification is described as epigenetic and can be heritable/reversible

## Key terms

- [epigenetic modification](https://www.gradenine.co.uk/glossary/epigenetic-modification)
- [histone](https://www.gradenine.co.uk/glossary/histone)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-epigenetic-modifications-to-histone-1e484915) · Published by Druglandscape Ltd.