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Understanding DNA Replication Process and Mechanism

Discover the intricate process of DNA replication - separating, replicating, and repairing DNA strands. Explore the semiconservative model, replication bubbles, leading vs. lagging strands, and the crucial repair mechanisms. Learn about DNA polymerases, primers, and ligases in this essential genetic process.

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Understanding DNA Replication Process and Mechanism

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  1. DNA Replication May, 2013

  2. DNA Replication • Genetic information needs to be replicated before cell division (mitosis) • Mitotic cell division is important for tissue growth, embryonic development and tissue regeneration

  3. 3 Proposed Models

  4. Meselson and Stahl

  5. Semiconservative • DNA replication is semiconservative • Each DNA molecule is composed of a parent strand and a newly synthesized strand

  6. Mechanism of DNA Replication 1) SEPARATE 2) REPLICATE 3) REPAIR

  7. 1) Separate • DNA strands need to be unraveled and kept separate • DNA helicase – enzyme that unwinds the double-helix by disrupting H-bonds • Single-stranded binding proteins – bind to the single-stranded DNA to prevent annealing • DNA gyrase – relieves tension from the unwinding of DNA; cuts both strands and reseals the strands

  8. DNA gyrase Topoisomerase (swivels) DNA helicase Single-stranded binding proteins (SSB)

  9. Replication Bubbles • In eukaryotes, there are multiple sites of replication forming “bubbles” in the DNA • 1 replication site = 700 days to replicate • Multiple sites = 5-10 hours

  10. 2) Replicate • Primase – creates an RNA primer (10-60 base pairs) in front of the DNA template strand • The primer acts as a start sequence to direct replication; easy to remove • DNA polymerase III (DNAP3) – recognizes the primer and synthesizes DNA in the 5’ to 3’ direction by adding deoxyadenosine triphosphates

  11. Adding Bases • Free bases are found in the nucleoplasm • DNAP3 uses the energy from breaking the phosphates of the deoxyadenosine triphosphates to drive the process • The two phosphates are recycled

  12. Leading vs Lagging Strand • DNA is synthesized in the 5’ to 3’ direction • Leading strand – synthesized continuously • Lagging strand – synthesized in short fragments • Primers are added continuously • DNAP3 makes complementary segments called “Okazaki fragments” • DNA polymerase I (DNAP1) – removes primers • DNA ligase – joins the Okazaki fragments

  13. Lagging Strand

  14. 3) Repair • DNAP3 and DNAP1 proofread the newly synthesized strand • Both can function as an exonuclease (cuts out nucleotides) • They backtrack past the mistaken nucleotide, excises it and continues adding nucleotides to the complementary strand • First checkpoint

  15. Summary • http://highered.mcgraw-hill.com/sites/dl/free/0072437316/120076/bio23.swf • http://www.youtube.com/watch?v=-mtLXpgjHL0&feature=related

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