Structure of DNA

1
Structure of DNA
2
Nucleotides

• Know that DNA contained 4 different types of
  nucleotides
• Purines 2 ring bases
• Adenine
• Guanine
• Pyrimidines 1 ring base
• Thymine
• Cytosine

3
Chargaffs rules

• Found nucleotide content of DNA not fixed
• Variability between species
• All members of a species have same base
  composition
• of As of Ts, CG

4
Rosalind Franklin

• X-ray diffraction of DNA
• Showed DNA was helix
• Showed a feature was repeated over and over,
  hydrogen bonded bases.

5
Watson and Crick

• Using data from x-ray diffraction, constructed
  model of DNA in 1953,
• received Nobel Prize in 1962
• Sugar phosphate backbone on outside, base pairs
  inside.
• Complementary base pairing A with T, C with G
• Pyrimidine always with purine.

6
DNA Replication

• Copying of a DNA molecule
• Old DNA strands used as a template for new one
  new strand
• Semiconservative replication
• One old strand conserved in each new daughter
  molecule.

7
Replication

• Steps
• unwinding (unzipped) DNA helicase, breaking of
  weak hydrogen bonds
• Complementary base pairing bases are always
  available in nucleus
• Complementary nucleotides join to form new
  strands (DNA polymerase)
• DNA polymerase can also be used as a proofreader.

8
Meselson and Stahl 1958

• Replication experiment using cells grown in heavy
  and light nitrogen
• Results DNA replication is semiconservative

9
Prokaryotes versus Eukaryotes

• Prokaryotic replication
• Replication can occur in 1 or 2 directions
• Very fast, 40 minutes for entire chromosome
• Can happen concurrently
• Eukaryotic
• Begins at numerous origins along chromosome
• Spread bidirectional until they meet
• replication fork V shape when ever DNA is being
  replicated
• Slower but do cover significant DNA because of
  many origins of replication

10
Replication Errors

• Genetic mutation permanent change in sequence
  of bases
• 1 per 100,000 base pairs at most, after
  proofreading, 1 per billion
• Mutations can be result of DNA damage
• Environmental factors
• DNA repair enzymes exist and are somewhat
  effective.
• Remember Mutations are raw material for
  evolutionary process!!