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DNA Replication
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DNA Replication

DNA REPLICATION
  • Complementary
  • Semiconservative
  • Bidirectional
  • Occurs in the S phase
  • Synthesis in the 5' to 3' direction – antiparallel
KEY ENZYMES
  • Helicase separates DNA strands at replication fork
  • Topoisomerase relieves supercoiling downstream of replication fork
  • Primase synthesizes RNA primer
  • DNA polymerase synthesizes DNA
  • Ligase joins DNA fragments together
Replisome complex
  • Protein complex assembled at each replication fork
  • Comprises: helicase, primase, DNA polymerases, single-strand binding proteins
Key differences between Prokaryotes and Eukaryotes
• Eukaryotes have multiple origins of replication • Different DNA polymerases, helicases, topoisomerase and ligase • Eukaryotes have nucleosomes and telomeres • Eukaryotes typically have more DNA
Prokaryotic DNA Polymerases
• DNA pol III: synthesizes both the leading and lagging strands. • DNA pol I: removes RNA primers and fills the remaining gaps with DNA nucleotides.
Eukaryotic DNA polymerases
• DNA pol epsilon: elongates leading strand DNA. • DNA pol delta: elongates lagging strand DNA and displaces RNA primers from the lagging strand. • DNA pol alpha: complexes with primase to synthesize primers that comprise RNA and DNA.
PROKARYOTIC REPLICATION (E. COLI)
  • DNA is melted at one origin of replication (region rich in adenine and thymine)
  • 2 replication forks with replisomes on each side of origin are created (bidirectional replication)
  • Helicase separates parent DNA strand (breaks H-bonds)
  • Single-stranded DNA binding proteins stabilize strands (prevent degradation)
  • Primase synthesizes RNA primers (~ 10 nucleotides long)
  • DNA polymerase elongates strand (adds to 3' end only)
  • Leading strand: oriented 3' to 5' towards fork
  • Lagging strand: oriented 5' to 3' towards fork --> Okazaki fragments (discontinuous replication)
  • Different DNA polymerase removes primer & fills gap with DNA
  • DNA ligase joins Okazaki fragments
  • Topoisomerase relieves supercoiled DNA downstream of replication fork