• Nucleosomes are the?structural unit of DNA packaging?in eukaryotes that?facilitate supercoiling
• Within a nucleosome, DNA is wrapped around proteins called?histones
• The tails of histones can be chemically modified which can influence?whether a gene will be expressed or not
  • An?acetyl?group,?methyl?group or a?phosphate?group can be added
  • Chemical modifications can either?activate or deactivate genes?by making the gene more or less accessible to transcription factors
• Methyl groups can also be?directly added to DNA?to change the activity of a gene

Acetylation and methylation of histone tails

• Positively charged?lysine?(an amino acid) in histone tails binds to negatively charged DNA
  • This helps DNA to?coil tightly?around the histone protein core
• Adding an acetyl group (acetylation) to lysine?neutralises the charge, causing the DNA to be?less tightly wrapped
  • RNA polymerase and transcription factors can more easily access the DNA so?gene expression is stimulated
• Adding a methyl group (methylation) to lysine maintains the positive charge causing the DNA to be more?tightly wrapped?and therefore?inhibits transcription/expression

Acetylation of the Nucleosome

Methylation of DNA

• DNA methylation commonly involves the direct addition of a methyl group (-CH₃) to cytosine bases which can influence gene expression
• Methylation of DNA suppresses the transcription of the affected gene by inhibiting the binding of transcription factors
• Cells use this mechanism to lock genes in the ‘off’ position
• DNA methylation can be affected by many environmental, lifestyle or age-related factors

• The synthesis of mRNA occurs in?three stages:
  • Initiation
  • Elongation
  • Termination
• During?initiation, RNA polymerase binds near the promoter, causing the?DNA strands to separate?to form an open complex
• During?elongation,?RNA polymerase moves?along the antisense strand
  • RNA polymerase adds the 5‘ end of the free RNA nucleotide to the 3’ end of the growing mRNA molecule
  • Elongation occurs in a?5’ to 3’ direction,?synthesising a single strand of RNA
• Termination occurs when RNA polymerase reaches a?terminator sequence
  • Which triggers the?detachment of the polymerase enzyme?and?mRNA strand

The antisense strand of the DNA molecule is the one that is transcribed