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.