DNA Replication Flashcards
What are the 3 potential models for DNA replication?
Conservative, semi-conservative, and dispersive
What would Messelson and Stahl have seen if DNA replication proceeded like the conservative model?
After the first round of replication, one band for the heavy isotope at the bottom and one band for the light isotope at the top. After the second round, same thing
What would Messelson and Stahl have seen if DNA replication proceeded like the semi-conservative model?
After the first round of replication, one band in the middle that is a hybrid of light and heavy isotopes. After the second round, would have the hybrid band in the middle and a band of the light isotope higher up
What would Messelson and Stahl have seen if DNA replication proceeded like the dispersive model?
After the first round of replication, one band in the middle that is a hybrid of light and heavy isotopes. After the second round, would still have one band of the hybrid, but would be higher up as more light isotope got incorporated in
What is the origin of replication? How many are there?
Is an AT rich region where the strands separate and replication begins. One in bacteria, many in eukaryotes
Which direction does replication proceed in?
Bidirectional from Ori
What is the difference between the leading and lagging strands?
Leading strand has 3’ end exposed, and the new strand runs 5’ to 3’. Lagging strand has the 5’ end exposed and the new strand is made of discontinuous Okazaki fragments
Why is there a leading strand and lagging strand?
DNA polymerase can only add nucleotides in the 5’ to 3’ direction, and the lagging strand runs opposite to that
What are the 7 key proteins in E. coli DNA replication?
DNA polymerase 3, DNA polymerase 1, primase, helicase, topoisomerase, SSBs, DNA ligase
What is the function of DNA polymerase 3?
Does most of the DNA synthesis and also does some proofreading
What is the function of DNA polymerase 1?
Removes primers and fills in those gaps with DNA and proofreading
What is the function of primase?
Make RNA primers needed to start synthesis
What is the function of DNA helicase?
Break the hydrogen bonds via ATP hydrolysis, that keep the strands together and unwind the strands
What is the function of topoisomerase?
Relieve the tension ahead of the replication bubble, unwind supercoiled DNA
What is the function of SSBs?
Single strand binding proteins. They bind to single strand DNA and stop it from reannealing
What is the function of DNA ligase?
Connects the Okazaki fragments
What are the steps of DNA synthesis?
- Helicase unwinds the strands, topoisomerase stays ahead of it
- Primase adds a short RNA primer
- DNA polymerase 3 synthesizes nucleotides
- DNA polymerase 1 removes the primers and synthesizes DNA in their place
- DNA ligase connects adjacent fragments
How is replication different in eukaryotes?
Chromatin: histones and chromatin proteins have to be disassembled for replication then reassembled
Larger and more complex chromosomes: more proteins involved, many origins of replication
Linear chromosomes: telomeres at the ends of chromosomes
What are telomeres? What is the enzyme involved?
Telomeres are specialized sequences of DNA at the ends of eukaryotic chromosomes. The gap created when DNA polymerase 1 removes the primer isn’t filled like the internal gaps. Telomerase comes in and makes more of that sequence, until there’s another Okazaki fragment
