Download Structure of DNA and Replication

Structure of DNA
Nucleotides
• Know that DNA contained 4 different types
of nucleotides
• Purines – 2 ring bases
– Adenine
– Guanine
• Pyrimidines – 1 ring base
– Thymine
– Cytosine
Chargaff’s rules
• Found nucleotide content of DNA not fixed
• Variability between species
• All members of a species have same base
composition
• # of A’s = # of T’s, C=G
Rosalind Franklin
• X-ray diffraction of DNA
– Showed DNA was helix
– Showed a feature was repeated over and
over, = hydrogen bonded bases.
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.
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.
Replication
• Steps
1. unwinding (unzipped) DNA helicase,
breaking of weak hydrogen bonds
2. Complementary base pairing – bases are
always available in nucleus
3. Complementary nucleotides join to form
new strands (DNA polymerase)
4. DNA polymerase can also be used as a
proofreader.
Meselson and Stahl 1958
• Replication experiment using cells grown
in heavy and light nitrogen
• Results: DNA replication is
semiconservative
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
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!!