7.2.3 Transcription

• 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