Eukaryotic Chromosome Structure

Enzymes of DNA Replication

  1. DNA Helicases - These proteins bind to the double stranded DNA and stimulate the separation of the two strands.
  2. DNA single-stranded binding proteins - These proteins bind to the DNA as a tetramer and stabilize the single-stranded structure that is generated by the action of the helicases. Replication is 100 times faster when these proteins are attached to the single-stranded DNA.
  3. DNA Gyrase - This enzyme catalyzes the formation of negative supercoils that is thought to aid with the unwinding processIn addition to these proteins, several other enzymes are involved in bacterial DNA replication.
  4. DNA Polymerase - DNA Polymerase I (Pol I) was the first enzyme discovered with polymerase activity, and it is the best characterized enzyme. Although this was the first enzyme to be discovered that had the required polymerase activities, it is not the primary enzyme involved with bacterial DNA replication. That enzyme is DNA Polymerase III (Pol III). Three activities are associated with DNA polymerase I.  The 3' to 5' exonuclease activities of DNA Pol I are important for replication, but DNA Polymerase III (Pol III) is the enzyme that performs the 5'-3' polymerase function.  The three activities are:
    1. 5' to 3' elongation (polymerase activity);
    2. 3' to 5' exonuclease (proof-reading activity)
    3. 5' to 3' exonuclease (repair activity).
  5. Primase - The requirement for a free 3' hydroxyl group is fulfilled by the RNA primers that are synthesized at the initiation sites by these enzymes.
  6. DNA Ligase - Nicks occur in the developing molecule because the RNA primer is removed and synthesis proceeds in a discontinuous manner on the lagging strand. The final replication product does not have any nicks because DNA ligase forms a covalent phosphodiester linkage between 3'-hydroxyl and 5'-phosphate groups.