Chapter 4

• Regulation through modifying histone tails
  • Targets specific amino acids
    • Lysine, Arginine, Serine (less common)
  • Three methods
    • Methylation
    • Acetylation
    • Phosphorylation
  • Histone tail modification
    • Variant effects depending on method/amino acid
    • Examples
      • Acetylation of Lysinelysine removes the polaritypolarity.
        • Results in weaker nucleosome bonds and more sliding
      • Trimethylation on aan H3 Lysine -> more heterochromatin
    • Reader complex
      • Histone tails attract reader complexes that recruit other proteinsproteins.
    • Lysine acetylation is adding an acetyl group to the bottom (NH3^+NH₃⁺) side of lysinelysine.
      • Removes a lot of the polarity
    • Lysine methylation is adding a lot ofmany methyl groups where the hydrogen was on the nitrogennitrogen.
  • Reader-Writer complex
    • The writer complex can modify nucleosomes and histone tails.
    • Allows a reader to bind and recruit more writers sequentially.
  • “Eraser” complex
    • Removal of nucleosomes
    • Reverse the process through histone demethylase or histone deacetylase.
  • Barrier protein
    • Binds to DNA, preventing the spread of heterochromatin.
• Centromeres and nucleosomes
  • Heterochromatin at the centromere is different from other heterochromatin because it is a different variant of the H3 histone. It is called Centromere Protein-A (CENP-A).
    • This lacks modifications found on regular H3
    • Attracts other nucleosome proteins
    • Creates a denser nucleosome packing than normal to form the kinetochore.
• Chromatin to Chromosome
  • Creates loops on the sister chromatid
  • The chromosome scaffold is used almost as a cytoskeleton-esque frame for chromosomes.
• DNA to chromosome
  • Double-strand DNA
  • Nucleosome beads
  • Chromatin fibers
  • Chromatin loops
  • Chromosome