DNA replication and repair (Ch. 14.3 and 14.5)
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Before cell division, all DNA in the nucleus should replicate during what 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)
How does replication get started?
- -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.
How do mistakes get corrected?
- -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.
What can cause damages to DNA?
- -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.
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