Epigenetics (increased methylation and decreased acetylation)
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of DNA structure from Year 12. You can test your knowledge on this below.
What is the overall charge of DNA?
Negative, due to the negatively charged phosphate groups in the DNA nucleotides
What is a histone protein?
Proteins that DNA is wrapped / packaged around in eukaryotic cells.
Which bases pair together in DNA?
Adenine with Thymine, Cytosine with Guanine.
Topic Explainer Video
What is Epigenetics?
- Epigenetics is the study of heritable changes in gene expression without changes to the DNA base sequence itself.
- These changes involve chemical modifications to DNA or associated proteins (e.g., histones) that switch gene expression on or off.
What is the Epigenome?
- The epigenome refers to the chemical tags (modifications) that cover DNA and histones.
- It acts as a control system for gene expression, deciding which genes are active (expressed) and which are silenced.
Important: The epigenome is flexible and can be influenced by environmental factors like diet, stress, and toxins.
Mechanisms of Epigenetic Control
There are two main mechanisms that alter gene expression in epigenetics:
DNA Methylation
-
Definition: The addition of a methyl group to a DNA molecule, usually to the cytosine bases.
-
Increased methylation inhibits transcription by:
-
Preventing the binding of transcription factors to the promoter region.
-
Attracting proteins that compact the DNA (making it inaccessible).
-
Increased methylation = inhibition of transcription = switching off gene expression.
Remember, if a transcription factor cannot bind to DNA at the promoter region, RNA polymerase cannot bind to initiate transcription.
-
Decreased methylation stimulates transcription by:
-
Allowing the binding of transcription factors to the promoter region.
-
Not attracting proteins that compact the DNA (making it more accessible).
-
Decreased methylation = stimulation of transcription = switching on gene expression.
Histone Acetylation
-
Definition: The addition of an acetyl group to histone proteins.
-
Increased acetylation stimulates transcription by:
-
Reducing the positive charge on histones.
-
Reducing the attraction between histone proteins and negatively charged DNA.
-
Causing the DNA to loosen from the histone proteins and become less tightly wrapped.
-
Allowing transcription factors to bind to promoter regions and RNA polymerase to initiate transcription.
-
Increased acetylation = Increased transcription = switching on gene expression.
-
Decreased acetylation inhibits transcription by:
-
Increasing the positive charge on histones.
-
Increasing the attraction between histone proteins and negatively charged DNA.
-
Causing the DNA to tighten around the histone proteins and become more tightly wrapped.
-
Preventing transcription factors from binding to promoter regions and preventing RNA polymerase from initiating transcription.
-
Decreased acetylation = Inhibits transcription = switching off gene expression
Summary: Methylation vs Acetylation
| Modification | Effect on Gene Expression |
| Increased Methylation (DNA) | Decreases transcription (gene silenced) |
| ncreased Acetylation (Histones) | Increases transcription (gene activated) |
| Decreased Methylation (DNA) | Increases transcription (gene activated) |
| Decreased Acetylation (Histones) | Decreases transcription (gene silenced) |
No answer provided.
Key Terms
- Epigenetics: Heritable changes in gene function without changes to the DNA base sequence.
- Epigenome: All chemical modifications on DNA and histones that regulate gene expression.
- DNA Methylation: Addition of a methyl group to DNA.
- Histone Acetylation: Addition of an acetyl group to histones.
- Chromatin: DNA and histone protein complex.
No answer provided.
Exam Tips
When explaining epigenetic control:
- Always state the effect on gene expression (increase or decrease).
- Specify whether DNA or histones are modified.
- If the question mentions increased methylation, link it clearly to inhibiting transcription by preventing transcription factor binding.
No answer provided.
Explain how DNA methylation and histone acetylation can inhibit the transcription of genes. (5 marks)
- DNA methylation involves adding a methyl group to DNA.
- Increased methylation prevents transcription factors from binding to DNA.
- Methylation also attracts proteins that condense the DNA, making genes inaccessible.
- Histone acetylation involves adding an acetyl group to histone proteins.
- Decreased acetylation causes the DNA and histone proteins to become more tightly wrapped, preventing transcription factors from binding to DNA.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!