DNA Synthesis Flashcards
What is the direction of DNA chain growth?
5’ to 3’
Where does DNA polymerase attach?
The 3’ end of the priming strand.
DNA primase
Synthesizes an RNA primer on lagging strand (10 nucleotides long).
How is the old RNA primer erased on a lagging strand?
By DNA polymerase, it just chews through it.
When does the synthesis of each okazaki fragment end?
When DNA polymerase runs into the RNA primer attached to the 5’ of the previous fragment.
DNA ligase
Joins 3’ end of new DNA fragment to the 5’ end of the previous, uses ATP
DNA helicase
Hydrolyzes ATP when bound to single stranded DNA.
Single stranded binding proteins
Prevent formation of short hairpin helices.
Sliding clamp
Keeps DNA polymerase bound until it reaches a sequence of double stranded DNA.
What is the purpose of the clamp loader?
Assembles the sliding clamp and DNA polymerase by ATP hydrolysis. It releases when DNA polymerase binds.
DNA topoisomerase
Reversible nuclease that adds itself covalently to a DNA backbone and breaks the phosphodiester bond. Creates a relative state of negative super coiling.
Topoisomerase I
Acts even when no DNA replication is occuring, cuts one strand.
How does all DNA synthesis begin?
With an RNA primer.
Topoisomerase II
Cuts two strands, creates a protein “gate”
When can eukaryotes perform DNA replication?
During the S phase.
What are the proteins that initiate replication in bacteria?
Initiator proteins, DNA helicase, helicase loading proteins. DNA primase, DNA polymerase and an RNA primer.
Why is there a pause between replications in bacterial DNA replication?
After initiation, the initiator protein is inactivated by hydrolysis and the origins are hemimethylated, making them resistant to initiation.
What happens in the G1 phase in eukaryotic replication?
Replicative helicases are loaded onto DNA next to the ORC to create a prereplicative complex.
What happens between the G1 and S phase in eukaryotic replication?
Kinases activate the helicases and prevent an assembly of new prereplicative complexes until next M phase.
What happens when a nucleosome passes by a replication fork?
The histone octamer is broken into an H3-H4 tetramer and H2A-H2B dimers.
What happens to H3-H4 tetramers after a broken nucleosome?
They remain loosely associated with DNA and are distributed at random to one or the other daughter complex.
What happens to H2A-H2B dimers after a broken nucleosome?
They are released completely
Telomerase
recognizes the tip of an exisiting telomere DNA repeat and elongates in 5’ to 3’ using RNA template. It is a reverse transcriptase.
How does telomerase work?
Adds an extra section of non useful DNA to the end so DNA polymerase can remove the RNA primer.
What is the RNA template carried by telomerase?
ACCCCAAC
What does shelterin do?
It hides telomeres from cells damage detectors that continually monitor DNA by forming a t loop at the end of a chromosome.
