MolBio5 - 36 Flashcards
What breaks down Ppi into 2Pi?
Pyrophosphatase
Why is DNA synthesis said to be irreversible?
It involves the breakdown of Ppi into 2Pi
What provides the energy for DNA synthesis?
Breakage of two high energy phosphate bonds
What is the energy usage of DNA replication?
-7kCal/mol
Describe the direction of DNA synthesis?
Bidirectional
What removes the RNA primers of the okazaki fragments?
Ribonuclease H
What makes the okazaki fragments continuous?
DNA ligase
What are the 9 replication proteins?
Helicase, single-stranded binding proteins, primase, polymerase, sliding clamp, clamp loader, ribonuclease H, ligase and topoisomerase
What does ribonuclease H do?
Removes RNA primers
What does DNA ligase do?
Uses the energy of ATP hydrolysis to ligate newly synthesise, adjacent DNA fragments in a two-step catalytic reaction
Outline the first step of DNA ligase
ATP + 5’-P > P-P + 5’-P-AMP
Outline the second step of DNA ligase
P-P > 2Pi + free energy
What is the ligation process described as?
Energetically highly favourable
What keeps the ligation process energetically highly favourable?
Phyrophosphatase
What does DNA primase do?
Synthesis a short RNA primer required by ALL DNA for synthesis initiation
What does DNA helicase do?
Uses ATP to separate the parental DNA strands at the replication fork and moves the fork forwards
What is DNA helicase activity reliant upon?
ATP and magnesium
What can mutations in helicase activity cause?
Progeria, such as Werner Syndrome, resulting in premature aging
Describe Werner Syndrome
A form of progeria, mutation are AR in RECQ helicase gene WRN
What aids DNA polymerase?
Sliding clamp and clamp loader
How do sliding clamp and clamp loader help DNA polymerase?
Sliding clamp is positioned close to the primer:template junction by clamp loader, encircling the DNA like a nut on a bolt to help move the DNA polymerase forwards
What do single-stranded binding proteins do?
Expose single-stranded DNA in the replication fork, making it available for templating synthesis of the new DNA strand and easing replication fork progression
What do DNA topoisomerases do?
Prevent DNA from becoming tangled during DNA replication
How do DNA topoisomerase prevent DNA from becoming tangled?
Nick and reseal the backbone of the parental helix to relieve superhelical tension
What do type I topoisomerases do?
Nick and reseal one of the 2 DNA strands, ATP independently
What do type II topoisomerases do?
Nick and reseal both of the DNA strands, ATP dependently
Describe replication initiation
Biphasic - replicator selection occurs in G1, then origin activation occurs in S phase
Outline origin activation
Unwinding of DNA and recruitment of DNA polymerase
What is temporal seperation in terms of replication control?
Ensures replicator selection and origin activation are seperated by a fixed amount of time, ensuring each chromosome is only replicated exactly once per cell cycle
Outline the mechanism of pre-replicative complex construction
Origin recognition complex (ORC) binds to replicator sequence, helicase loading proteins Cdc6 and Cdt1 binds to ORC, helicase Mcm2-7 binds to complete formation of pre-RC
How is the pre-replicative copmlex is controlled?
Low Cdk = pre-RC formation allowed, no activation; high Cdk = no pre-RC formation allowed, pre-RC activation
What is the end replication problem?
Need for an RNA primer for initiation of DNA synthesis needs to be removed, but eventually this would lead to chromosome shortening
What is added to DNA after replication, and why?
TTAGGG repeats by telomerase to compensate for the loss of telomere sequences caused by RNA primer removal
