epigenetics

• phenotype: the result of an interaction between genotype and the environment
• acetylation of histones affects gene expression

  acetyl groups (-COCH3) can be added to lysine amino acids on histone proteins

  lysine has a positively charged R group, this forms ionic bonds with the negatively charged phosphate backbone of DNA (this helps DNA to coil tightly around the histone protein core)
• acetylation of histones affects gene expression

  adding acetyl groups (acetylation) to lysine residues removes positive ion and therefore removes a bond between the histone protein and the DNA, this causes the DNA to be less tightly wrapped
• what happens when the DNA is less tightly wrapped? (acetylation)

  RNA polymerase and transcription factors bind more easily and therefore gene expression can occur

  the gene is said to be activated
• what is the result of deacetylation?

  returns lysine to its positively charged state which has a stronger attraction to the DNA molecule and therefore inhibits transcription and once again stops the gene from being expressed
• DNA methylation

  commonly involves the direct addition of a methyl group to cytosine bases which can influence gene expression
• what does methylation of DNA result in

  suppresses the transcription of the affected gene by inhibiting the binding of transcription factors and enzymes needed for transcription (e.g RNA polymerase)

  cells use this mechanism to lock genes in the 'off' position, gene is repressed/inactivated
• DNA methylation can be affected by many environmental, lifestyle or age-related factors
• epigenome is heritable i.e when a cell divides and replicates, epigenetic changes affecting the expression of genes in the DNA of that cell may be passed on to daughter cells
• during gamete production, DNA in the parent cell usually undergoes de-methylation but often methyl groups are not removed and therefore are present in the DNA on the sperm/egg cells 


  this happens because if an epigenetic change occurs in response to an environmental factor, it may be beneficial for this epigenetic change to also occur in daughter cells (or gametes) so that they are also better adapted for the environmental factor (in the same way the parent cell was)
• mounting evidence demonstrates that modifications to the epigenome in one generation can be passed on to the next generation at the cellular or whole organism level