 Helicase enzyme binds to the replication initiation site and begins to unwind and separate the DNA helix into single strands

 The hydrogen bonds between strands are broken creating a replication bubble  Replication Fork is the point where the two parent strands join

 As the helicase continues to unwind DNA a second enzyme called topoisomerase or gyrase relieves the pressure building up on the molecule

 Single-stranded binding proteins anneal to the newly exposed template strands, preventing them from reannealing

 Primase binds to the exposed single stranded DNA and creates an RNA primer sequence 10 bases long  This sequence permits the binding of DNA Polymerase to the strand which begins elongation of the new strand  This is an endergonic reaction requiring the input of energy  Note: each of the bases is a tri-phosphate – energy is released when they bond.

 DNA Polymerase can only function in the 5’ to 3’ direction  It will add bases to the exposed 3’ end of the growing DNA strand  Since the parent DNA is made up of two anti- parallel molecules, one strand will be elongated continuously the other in smaller fragments

 Begins at the ORI  Binding of gyrase and helicase starts the process of separating the parent strands  SSBs stabilize the parent molecules and prevent re-annealling  Primase places a primer RNA molecule on the parent strand – 10 bases long with exposed 3’ hyroxyl  DNA polymerase III binds to primer and synthesizes copy

 DNA polymerase I will bind to RNA primer, cut it out and replace it with DNA nucleotides  DNA ligase will then stitch together the fragments to create one continuous strand  Both DNA polymerase I and DNA polymerase III can “proof-read” the DNA ensuring that the right base pairs are made to the parent strand  A pairing with T  G pairing with C