DNA Replication
DNA REPLICATION
- Complementary
- Semiconservative
- Bidirectional
- Occurs in the S phase
- Synthesis in the 5' to 3' direction – antiparallel
- 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