W2L4 - DNA packaging

• packaging of DNA:

  • each human cell contains nearly 2m of DNA but the nucleus is only 6μm
  • is accomplished by special proteins that bind to the DNA and fold it
• Eukaryotic DNA packaging:

  • packaging occurs in chromosomes
  • In humans, 3.2 ✕ 10^9 nucleotides are divided between 46 chromosomes
  • The human genome, 3.2 ✕ 109 nucleotides = 24 chromosomes (22 autosomes, X and Y)
  • Each chromosome is a single enormously long linear DNA molecule (average size 140 Mb) associated with proteins
  • Proteins are divided into “histones” and “non-histone chromosomal proteins”
  • DNA + protein complex = Chromatin

  
• The nucleosome:

  • core histones of the nuclosome are surrounded by 146 base pairs of DNA
  • DNA wraps 1.65x around the histone core

  
• String of beads:

  • adjacent nucelosomes connect via spacer DNA (H1 histone attaches here)

  
• Chromatin fibre:

  • string of beads is further coiled to form chromatin fibre (30nm diameter)
  • Four nucleosomes come together to form B (the fourth is behind the bottom nucleosome)
  • They stack as a zig-zag

  
• Histone 1:

  • it plays an important role in stacking the two nucleosomes to form a chromatosome

  
• Packaging:

  • steps
  • Start with DNA
  • DNA wrap around histones to form nucleosomes
  • Nucleosomes stack to form chromatin
  • Chromatin is then further condensed by addition of scaffold proteins (exact method not known)
  • DNA not always tightly packaged as seen in dividing cells
  • Most chromosomes in non-dividing cells are too fine and tangles to be clearly visible

  
• Condensed chromosomes

  • Chromosomes condense during cell division to give classic X shape
  • G banded metaphase karyogram
  • Short arm = p
  • Long arm = q
  • Numbered subdivisions
  • Non dividing cells - interphase
  • Interphase chromosomes are visible in some insect cells
  • Endoreplication - DNA synthesis without cell division
  • Chromosomes are highly organised in interphase
• Interphase - types of chromatin

  • Euchromatin
  • Less condensed chromatin
  • Doesn't stain
  • Heterochromatin
  • Highly condensed and visible under light microscopy
  • Gene activity is suppressed

  
• Summary of packaging:

  A) centromere

  B) double helix

  C) nucleosomes