Eukaryotic Chromosome Structure
Enzymes of DNA Replication
DNA Helicases - These proteins bind to the double stranded DNA and
stimulate the separation of the two strands.
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.
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.
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
5' to 3' elongation (polymerase activity);
3' to 5' exonuclease (proof-reading activity)
5' to 3' exonuclease (repair activity).
Primase - The requirement for a free 3' hydroxyl group is fulfilled
by the RNA primers that are synthesized at the initiation sites by these
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