DNA Replication: The Three-act Play

Overview
(from http://www.teacherweb.com/CA/NogalesHighSchool/mespinoza/h4.aspx)

ACT 1: Initiation

• Single-strand binding proteins enter to stabilize the template strands, keeping them apart during replication.
• Helicases unwind the template DNA strands. They keep working at the replication fork to continuously open up the bubbles. 
• To release the tension, as the bubbles opening up, gyrases cut the DNA, afterwards, put it back.

ACT 2: Elongation

• RNA primase enzymes begin the process by building a small complementary RNA segment called RNA primers to the origin of the strand, therefore, DNA polymerase III can add DNA nucleotides to the RNA primer. 
• Nucleotides on the daughter strand can only elongate in 5' to 3' direction. This strand is called leading strand.
• On the opposite strand, which is called lagging strand, DNA polymerase III is moving away from the replication fork.
• On lagging strand, okazaki fragments are formed with series of RNA primers and short DNA fragments added by DNA polymerase III.

ACT 3: Termination

• DNA polymerase I enter. They remove the RNA nucleotides at the primers with DNA nucleotides.
• DNA ligase catalyzes the reaction of  forming bonds between short DNA strands on the lagging strand.
• DNA polymerase I proofread the daughter DNA.