8.2.7 Epigenetics

Mechanisms of Epigenetic Control

1. DNA Methylation:

• Methyl groups (-CH3) are added to cytosine bases in DNA.
• This process usually occurs at CpG sites (cytosine followed by guanine).
• Methylation makes DNA more tightly packed, preventing transcription factors from binding.
• Effect: Genes become silenced or switched off.

2. Histone Modification:

• Histones are proteins around which DNA is wrapped.
• Acetylation: Adding acetyl groups (-COCH3) to histones makes DNA less tightly packed, allowing transcription.
• Increased acetylation activates genes (switches them on).
• Deacetylation: Removing acetyl groups makes DNA tightly packed, preventing transcription.
• Decreased acetylation silences genes (switches them off).

3. Non-Coding RNA (ncRNA):

• Some non-coding RNA molecules can interfere with gene transcription by binding to DNA or histones, influencing gene expression.

Examples of Environmental Influences

• Diet: Nutrients such as folate can influence DNA methylation patterns.
• Stress: Prolonged stress can lead to changes in histone acetylation.
• Toxins: Chemicals like cigarette smoke can affect epigenetic markers, potentially increasing disease risk.

Epigenetics and Inheritance

• Epigenetic changes can sometimes be passed on to offspring.
  • Example: Studies have shown that malnourished mothers may pass on changes in gene expression to their children, influencing metabolism.
• However, during gamete formation, most epigenetic markers are removed through a process called epigenetic reprogramming, though some changes may escape this and be inherited.

Epigenetics and Disease

• Cancer: Abnormal DNA methylation can switch off tumour suppressor genes, contributing to cancer development.
• Autoimmune Disorders: Misregulation of epigenetic markers can lead to overactive immune responses.