DNA

Features of DNA

• Double stranded
• Antiparallel strands
• Right-handed helix
• Sugar-phosphate backbone
• Bases on the inside
• Stabilize H-bonding
• Specific base pairing
• About 10 nucleotides per helical turn

DNA Structure

• Chargoff’s rule
  • A pairs with T
  • G pairs with C
  • Keeps width consistent
• Complementary DNA strands
• Antiparallel DNA strands
  • One strand goes 5’ to 3’
  • The other strand goes 3’ to 5’

Major and Minor Grooves

• Grooves are revealed in the space-filling model
• Major groove: proteins bind to affect gene expression
• Minor groove: narrower

Molecular Structures of Eukaryotic Chromosomes

• Typical eukaryotic chromosome may be hundreds of millions of base pairs long
  • Length would be 1 meter
  • But must fit in cell 10 to 100 micrometer
• Chromosome
  • Discrete unit of genetic material
• Chromosomes composed of chromatin
  • DNA-protein complex

Eukaryotic Chromosome

• Dependent on the cell cycle
• In interphase chromatin is less condensed
• In mitosis chromosomes condense 10,000-fold and form distinct structures.
• Overall organization of a eukaryotic chromosome is greater than the organization of the prokaryotic chromosome.

Model of Chromosome Structure

• First level of packing
  • Nucleosome
• 2nd level coiling of the beads into a helical array - 30nm fiber = chromatin
  • Requires additional proteins to those in the basic particles
• 3rd level non-histone chromosomal proteins form scaffold
  • Packing of fiber itself (approx. 10,000-fold in mitotic chromosomes)

Nucleosome

• Histone octomer + DNA = nucleosome
• 146 nucleotides + 2H2a + 2H2b + 2H3 + 2H4 (histone octomer)
• Electrostatic action between the positive charge histone and the negative phosphates of the DNA are an important stabilizing force in maintaining chromatin structure
• Nucleosomes joined by linker DNA (~80 bps) and histone H1 to form chromatin fiber (“Beads on a String”)

Radial Loop Domains and Chromatin

• Radial loop domains: interaction between 30-nanometer fibers and nuclear matrix
  • Each chromosome located in discrete territory

Types of Chromatin

• Euchromatin: DNA that is undergoing normal packing. Genes in these regions can be expressed
  • Appx. 90% of the DNA in the cell.
• Heterochromatin: highly condensed in comparison to euchromatin. Genes in these regions are not expressed.

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