The Molecular Process of DNA Replication Flashcards
How accurate is DNA polymerase?
It only makes 1 error in every 10 million nucleotide pairs it copies
How is DNA polymerase so accurate?
It has error correcting activity -> proofreading
What does DNA pol do before adding a new nt?
It checks the previous nt to see if its correct
What happens if DNA pol finds out the previous nt is incorrect?
(3)
DNA pol removes the incorrect nt by cutting the phosphodiester bond
This releases the nt
DNA pol tries again to add the correct nt
What is DNA polymerase III?
The main replicating enzyme in bacteria
What does DNA polymerase III do?
2
DNA polymerase activity in forward direction
Exonuclease activity in reverse - proofreading
What does DNA polymerase I do?
Replaces RNA in primer with DNA
In the forward direction exonuclease activity mediating nick translation during DNA repair
What does DNA polymerase I do in the forward direction?
2
DNA polymerase activity
Exonuclease activity - nick translation during DNA repair
What does DNA polymerase III do in the reverse direction?
Exonuclease activity - proofreading
What is an exonuclease?
An enzyme which removes successive nucleotides from the end of a polynucleotide molecule
What are the four key players in DNA replication?
Ligase
Helicase
Primase
DNA polymerase
What does Primase do?
Adds the RNA primers
What does helicase do?
Unzips the helix
What does ligase do?
Glues the Okazaki fragments together
What needs to be done to start a new DNA strand?
Primase adds short segments (5-10 nts) of RNA bases first to provide a base-paired 3’ end as a starting point for DNA pol
How many primers are needed on the leading strand?
Only one
How many primers are needed on the lagging strand?
New primers are needed continuously
What three additional enzymes are needed on the lagging strand?
Nuclease
DNA polymerase I (bacteria only)
DNA ligase
What does nuclease do?
Breaks apart the RNA primer
What does DNA polymerase I do in bacteria?
Replaces RNA with DNA
What does DNA ligase do?
Joins 5’ phosphate end of one DNA fragment to 3’ OH end of the next (ATP required for ligase activity)
What four proteins cooperate to form the replication machine?
Helicase
Topoisomerase
Sliding clamp
Single strand binding proteins
Write a note on helicase.
3
An enzyme
Uses energy of ATP hydrolysis to speed along DNA and separate the strands of parental DNA ahead of polymerase
It breaks the hydrogen bonds between base pairs
Write a note on topoisomerase.
2
An enzyme
Prevents DNA helix upstream from becoming supercoiled by relaxing it
Write a note on a sliding clamp.
2
Keeps DNA polymerase firmly attached to DNA template
On the lagging strand it releases polymerase from the DNA each time an Okazaki fragment is completed
