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Transcript
Topic 2.7 DNA Replication Why does DNA replicate? • All cells must replicate DNA before mitosis/meiosis • All daughter cells receive the correct chromosomal (genetic) information When does DNA replicate? • DNA Synthesis (SPhase) Replication • All cells must replicate DNA before mitosis or meiosis How does DNA replicate? • Three possible modes of DNA replication are possible: • conservative • semiconservative • dispersive Meselson-Stahl experiment • DNA replication is semiconservative • each new DNA molecule consists of one old strand and one newly synthesized strand Meselson-Stahl experiment • DNA replication is semiconservative • each new DNA molecule consists of one old strand and one newly synthesized strand Semiconservative Replication • The complementarity of DNA strands allows each strand to serve as a template for synthesis of the other Where does DNA replication start? • begins at the origin of replication • bidirectional • replication fork DNA Synthesis in Bacteria • DNA polymerase catalyzes DNA synthesis and requires a DNA template and all four dNTPs DNA Synthesis • Chain elongation occurs in the 5' to 3' direction by addition of one nucleotide at a time to the 3' end • As the nucleotide is added, the two terminal phosphates are cleaved off What Occurs During DNA Replication? • There are seven key issues that must be resolved during DNA replication: • unwinding of the helix • reducing increased coiling generated during unwinding • synthesis of a primer for initiation • discontinuous synthesis of the second strand • removal of the RNA primers • joining of the gap-filling DNA to the adjacent strand • proofreading What Occurs During DNA Replication? Primer for polymerization • To elongate a polynucleotide chain, DNA polymerase requires a primer • The enzyme primase synthesizes an RNA primer replication fork • leading strand • continuous DNA synthesis • lagging strand • discontinuous DNA synthesis discontinuous DNA synthesis • lagging strand • RNA primer - Okazaki fragments • primers removed - DNA polymerase I • fragments joined - DNA ligase