DNA replication and repair (Ch. 14.3 and 14.5)

S-phase

DNA double helix opens up, and each strand is copied onto a new DNA strand, using old strands as templates. Newly formed helices have one old and one new strand.

• -DNA synthesis is catalyzed by this enzyme.
• -Adds nucleotides to the 3' end of the growing DNA strand.
• -DNA synthesis always from 5' to 3' direction.
• -Polymerase doesn't add nucleotides until there's an open 3' end with hydroxyl group (-OH)

• -Double helix splits apart in some place.
• -Appear as bubbles

• -Moves along the opening unzipping the double helix, expanding the replication bubble in both directions.
• -Single strand DNA binding proteins bind to the open region of the helix to keep the strands joining again.

• -Enzyme that adds a short RNA fragment that serves as a primer with an open 3' OH group.
• -DNA polymerase starts adding nucleotides to make the new strand.

Strand that is synthesized in the direction of helix opening

• Strand synthesized in the direction away from the helix opening.
• -Synthesized in fragments, known as Okazaki fragments.
• -When second fragment runs into first, RNA primer is destroyed and space filled with DNA.
• -DNA ligase joins the two fragments.

• -DNA polymerase has the ability to proofread its own work.
• -If wrong nucleotide is added by mistake, it backs up, removes the wrong nucleotide, and adds the correct one.
• -If mistakes left behind by DNA polymerase, there's other proof reading enzymes to correct them.

• -Chemicals
• -poisons
• -harmful rays
• -and natural aging process

• When damage is recognized, it cuts off the damaged area, and makes a new DNA strand to fill the gap.
• DNA ligase joins the new piece of DNA with old.