epigenetic changes

• when chemical groups are added to DNA or histone proteins
• methyl groups 


  added to DNA

  switches genes off

  RNA polymerase cannot bind to the promotor region
• Acetyl groups (when added)


  bind to histone proteins

  loosen coils of DNA

  RNA polymerase has access to the gene

  gene is switched on
• Acetyl groups (when removed)


  DNA is tightly coiled

  RNA polymerase cannot bind to gene

  gene is switched off