3 - DNA Replication and Repair Flashcards
Where does replication begin?
replication origins
what protein is used to stop the base pairs from annealing?
single-stranded binding proteins (SSBs) bind to the exposed DNA single strands and block hydrogen bonding.
Helicase
joins at a replication origin and begins unzipping the DNA (breaking H-bonds), creating a replication fork.
Topoisomerase
attaches to the DNA strand ahead of the helicase and relieves the tension in the DNA by cutting and re-bonding the backbone.
In eukaryotes, how many origins of replication are there (not a number)
multiple origins of replication moving in both directions, creating replication bubbles that work towards each other
Nucleotides are joined by enzymes called…
DNA polymerases 3
explain how DNA polymerases does its job
binds to the template strand and adds complementary nucleotides
Polymerases can only add nucleotides to the 3’ end of an existing DNA (or RNA) strand.
A new strand is always assembled in the __ to __ direction
5’ to 3’ direction.
RNA primer
Polymerase can only add nucleotides to an existing strand, therefore an RNA primer needs to be added first
RNA primase
builds a small complementary RNA segment of about 10-60 base pairs on the strand at the beginning of the replication fork.
leading strand
moving toward the replication fork
builds a new DNA strand in a continuous way
lagging strand.
moving away from the replication fork builds a new DNA strand in a fragmented way because it needs new RNA primers to be added as the parent strands continue to separate.
short DNA segments found between RNA primers are called
Okazaki fragments
DNA polymerase I
removes the RNA nucleotides one at a time and replaces them with DNA nucleotides
DNA ligase
forms the phosphodiester bond that joins the ends of DNA that make up the Okazaki fragments.
True or false, DNA polymerase enzymes also proofread and correct errors
true (DNA polymerase III can’t move if base pairs are mismatched)
what happens when there’s an error?
It usually backs up, replaces the incorrect base with the correct one, and then continues on.
Special DNA repair mechanisms read the strands for errors that might have been missed.
These repair mechanisms are complexes of proteins and enzymes, including DNA polymerase I and DNA polymerase II.
The resultant gap is filled in by a DNA polymerase III and completed by DNA ligase.
