DNA Replication

Act I: Initiation

• Helicase unwinds the double-stranded DNA at locations called replication origins. The structure that is created is known as "replication fork".

• Single-strand-binding-proteins then help to stabilize the newly unwound single strand, and prevent it from rebinding with its other strand.

• As the DNA unwinds, tension starts to build up on either sides of the unwound strand. Gyrase then comes in and cuts the two ends to release the tension.
• An RNA primer is added on to the DNA template strands by the enzyme primase to initiate the DNA replication process.

Act II: Elongation

• DNA polymerase III starts adding new nucleotides to the end of the RNA primers to create a new DNA daughter strand complementary to the original strand.

• SInce DNA is antiparallel, the new strands will grow in the opposite direction of each other.
• Leading strand: synthesized continuously in the 5'->3' direction by polymerase III. Only one RNA primer is needed to initiate the replication.

• Lagging strand: synthesized discontinuously in the 3'->5' direction by polymerase III. In the lagging strand, the RNA primase adds many RNA primers to form short, discontinuous fragments known as Okazaki fragments.

Act III: Termination

• Polymerase I comes in and now proofreads the lagging strands and replaces the RNA primers with DNA nucleotides.
• As a polymerase III reaches a previous RNA coded section, the Okazaki segments are glues together with ligase.

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