TR

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.