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DNA Replication. Chapter 16. Review: History & Structure. Griffith’s experiment. Hershey & Chase’s Experiment. Avery, M c carty , & Macloed’s Experiment. Wilkins & Franklin’s Experiment. Chargaff’s Experiment. Watson & Crick. DNA Replication. Making DNA from existing DNA

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dna replication
DNA Replication

Chapter 16

dna replication1
DNA Replication
  • Making DNA from existing DNA
  • Semi-conservative
    • At the end of DNA replication, each daughter molecule has one old strand (from the parent DNA) and one new strand (synthesized during replication)
dna replication2
DNA Replication
  • Step 1:
    • Helicases unwind DNA at origin of replication by breaking hydrogen bonds between nitrogen bases
    • Replication bubble forms as two parental strands separate
    • Replication fork forms at end of each replication bubble
dna replication3
DNA Replication
  • Step 2:
    • Single-strand binding proteins hold theunpaired DNA strands apart while new DNA strands are being synthesized
    • Topoisomerase protein binds to parental DNA to relieve strain untwisting puts on replication fork
dna replication4
DNA Replication
  • Step 3:
    • Primase creates a short RNA primer that binds to the parent DNA to signal DNA polymerase III where to begin adding nucleotides
    • RNA primer will later be replaced with DNA nucleotides

Step 4:

    • DNA Polymerase III adds nucleotides to exposed bases in 5’-3’ direction at the RNA primer
    • Leading strand
      • Produced continuously in 5’-3’ direction
      • Elongation moves towards replication fork
    • Lagging strand
      • Produced in pieces
        • Okazaki fragments
      • Elongation moves in opposite direction of replication fork (5’-3’)

Step 5:

    • Lagging strand is completed as DNA ligase seals Okazaki fragments
proofreading repair
Proofreading & Repair
  • Initial error rate in replication is 1 in 100,000 nucleotides
  • DNA polymerases proofread and correct errors
    • Error rate in completed replication is 1 in 10 billion bases
mismatch repair
Mismatch repair
  • For that 1 in 10 billion errors that escapes DNA polymerase or are due to environmental mutations
  • Many enzymes involved
  • Cut out damaged section (nuclease)
  • Replace with new nucleotides (DNA polymerase)
  • Seal in place (DNA ligase)
  • As cells divide, chromosomes erode after multiple DNA replications
  • Telomeres
    • End caps of non-coding DNA that protect the coding genes on the chromosomes
      • Think of the plastic pieces on the ends of your shoelaces
    • These sequences shorten after each round of replication so the actual genes do not
    • Part of aging process?
  • Enzyme that lengthens the ends of telomeres in germ cells (give rise to gametes)
  • Prevents loss of critical genes in zygote
  • Cancer cells have mechanisms that activate telomerase to allow unlimited division without loss of DNA
  • Bioflix animation
  • DNA replication cartoon