A2-LEVEL AQA BIOLOGY NOTES 
TOPIC 3: regulation of transcription and translation

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Gene transcription is mediated by the movements of transcription factors from the cytoplasm into the nucleus. While peptide hormones bind to a cell surface receptor and trigger the second messenger cascade, steroid hormones initiate this process by entering the cell itself.
Action of oestrogen

• Steroid sex hormone
  
• Enters cytoplasm via cell surface membrane
  
• Binds to receptor in cytoplasm
  
• Receptor-hormone complex passes through nuclear envelope and enters nucleus
  
• Complex binds to receptor bound to DNA
  
• Transcription activated at point of binding
  
• Protein synthesis, cell proliferation
  
• Oestrogen becomes dissociated from its receptor and leaves the nucleus/cell
  

Epigenetics = heritable changes in gene function, without changes to DNA sequence

• Caused by environmental factors
  
• Increased methylation of DNA or...
  
• Decreased acetylation of associated histones
  
• Thought to be involved in the development of cancer
  
  • Carcinogens affecting cell growth gene epigenetics
    
  • Silence tumour suppression genes
    
  • Activate oncogenes
    
• Can be used to treat some cancers
  
  • Pharmaceuticals designed to reverse epigenetic changes leading to cancer
    
  • De-methylation
    

RNA interference

• “RNAi”
  
• Process whereby RNA molecules act to inhibit gene expression
  
• Usually achieved by destroying mRNA
  
• Involves micro RNA (miRNA) and small interfering RNA (siRNA)
  
• Occurs in all eukaryotes and some prokaryotes