Epigenetics (AQA A Level Biology): Revision Note

Epigenetics

• Epigenetics involves changes in gene function, without changes to the base sequence of DNA

• All of the chemical modifications to all histone proteins and DNA (except base changes) in an organism are called the epigenome

  • In eukaryotic cells, the DNA in the nucleus is wrapped around proteins called histones

  • Histone proteins can be chemically modified through the addition of acetyl groups

  • DNA can also be chemically modified through the addition of methyl groups without changing the base sequence, which also leads to the regulation of gene expression

• The epigenome, like the genome, is heritable but can undergo change

  • Identical twins become more distinguishable with age because, despite having the same DNA, their epigenomes change independently, leading to differences

• Changes to the epigenome are caused by changes in the environment

  • Smoking, stress, exercise and diet can cause epigenetic changes

  • Internal signalling from the body's cells can also cause modifications to occur

• The chemical modification of histones and DNA controls how tightly the DNA is wound around them, as the intermolecular bonding between the histones and DNA changes

  • If the DNA is wound more tightly in a certain area, the genes on this section of DNA are 'switched off' as the gene and promoter regions are more hidden from transcription factors and RNA polymerase

• The modification of histones is reversible and therefore can be different in different cell types and can vary with age

Acetylation of histones

• Acetyl groups (COCH₃) can be added to lysine amino acids on histone proteins

  • Lysine has a positively charged R group, which forms ionic bonds with the negatively charged phosphate backbone of DNA

  • Adding acetyl (acetylation) to lysine residues removes the positive ion and therefore removes a bond between the histone protein and the DNA, which causes the DNA to be less tightly wrapped

• When the DNA is less tightly wrapped, RNA polymerase and transcription factors can more easily bind, and therefore, gene expression is stimulated

• Removal of acetyl (deacetylation) returns lysine to its positively charged state, which has a stronger attraction to the DNA molecule and therefore inhibits transcription

Methylation of DNA

• Methyl groups (CH₃) can be added to a carbon molecule on cytosine bases within sequences that contain multiple cytosine and guanine bases

• The addition of methyl groups (methylation) suppresses the transcription of the affected gene

  • This happens because the methylated bases attract proteins that bind to the DNA and inhibit transcription