Multiple enzymes and proteins are required for DNA replication at a replication fork.• These include DNA polymerases, single-strand DNA binding proteins, helicases, primase,topoisomerases, and DNA ligase. Some of these are multisubunit protein complexes.
- DNA polymerase III• It catalyzes the chemical reactions for polymerization of nucleotides.• DNA polymerase III begins synthesizing DNA in the 5’ 3’direction, beginning at the 3’ end of each RNA primer.• The newly synthesized strand is complementary and antiparallel to the parental strand used as a template.
- Single-stranded DNA binding protein (SSB) binds to the single-stranded portion of each DNA strand, preventing the strands from reassociating and protecting them from degradation by nucleases.• gp32, the most studied SSB protein, binds in a strongly cooperative fashion to single-strand DNA.•
- Helicase uses energy from the ATP to break the hydrogen bonds holding the base pairs together.• This allows the two parental strands of DNA to begin unwinding and forms two replication forks.•
- Primase is an enzyme that copies a DNA template strand by making an RNA strand complementary to it.• Primase synthesizes a short (about 10 nucleotides) RNA primer in the 5’ 3’ direction.• The parental strand is used as a template for this process.• RNA primers are required because DNA polymerases are unable to initiate synthesis of DNA, but can only extend a strand from the 3 end of a preformed “primer”•
- DNA ligase seals the "nicks" between Okazaki fragments, converting them to a continuous strand of DNA.• Covalently closes nicks in double-stranded DNA.