Module 8: V5 - V8 Flashcards
How does DNA synthesis work?
by adding nucleotides at the 3’ end of the new strand resulting in the formation of a phosphodiester bond
In which direction does DNA synthesis occur?
5’ to 3’
Which end of a primer is the polymerization reaction going to start during DNA synthesis?
at the 3’ end
In which direction does DNA synthesis occur on the leading strand?
towards the replication fork (continuous)
In which direction does DNA synthesis occur on the lagging strand? What are the implications of this?
away from the replication fork
polymerase starts and stops DNA synthesis resulting in the formation of short fragments which must be joined together (discontinuous)
What is the role of helicase?
opens the replication fork
How is the lagging strand prevented from binding to the leading strand?
the presence of single stranded binding proteins on the lagging strand
What is the name of short fragments produced on the lagging strand during DNA synthesis?
Okazaki fragments
What is primase involved in?
the formation of RNA primers on the lagging strand
What is DNA polymerase III involved in?
synthesis of the lagging and leading strand
What is DNA polymerase I involved in?
removes the primer and fills in the section with DNA
What is DNA ligase involved in?
forms a bond between the newest DNA and the DNA of fragment 1 as DNA polymerase 1 is unable to do this
How are enzymes on the lagging strand able to move in the direction of the replication fork?
DNA becomes looped around so that they can all travel together
What is the role of a beta-sliding clamp?
assists DNA polymerase on the leading strand so that it can travel for a long time
What is the role of the clamp loader?
joins to DNA polymerase enzymes
What happens when the primase comes in and produces the next primer?
the clamp-loader has a clamp ready to go and bends around to where the primer has been added and adds that beta-clamp onto the DNA with the primer
What happens when a beta-clamp is released and the polymerase has come off after completing the Okazaki fragment?
a new DNA polymerase binds to the next beta-clamp to produce the next Okazaki fragment
What is the role of a centromere found in a chromosome?
important for moving the chromosomes within the cell during mitosis or meiosis
What are the roles of telomeres found in a chromosome?
important for protecting the ends of the chromosomes
Why is replication of the end of a DNA molecule a problem for eukaryotes?
all the way along the lagging strand RNA primers are replaced by DNA until the end of the molecule where the primer can be removed but cannot be replaced with DNA because no 3’ end is available
How is the issue associated with replication of the end of a DNA molecule fixed?
telomeres prevent chromosomes getting shorter and shorter during DNA synthesis
Which enzyme add telomeres onto chromosomes?
telomerase
What does telomerase synthesise?
a very unusual DNA structure at the end of eukaryotic chromosomes
How accurate are DNA polymerases?
extraordinarily accurate, one mistake every 10^9 - 10^10 nucleotides (in E. coli, one error every 1,000-10,000 replications)
How is DNA polymerase so accurate?
utilises two mechanisms which are binding of the nucleotides in the active site and proofreading
What happens if DNA polymerase mechanisms fail?
DNA repair may be able to fix errors in the DNA sequence later
Which enzyme is involved in proofreading?
3’ -> 5’ exonuclease which removes nucleotides
How does 3’ -> 5’ exonuclease remove incorrect base pair matchups?
mismatched C=T blocks polymerase active site (AS) -> polymerase shifts around so C=T pair is moved into the exonuclease AS which removes the mispaired dC allowing the polymerase to shift back with a free AS ready for addition of the correct nucleotide
Which DNA polymerases have 3’ -> 5’ exonuclease activity?
both polymerase I and III
What is the unique exonuclease activity of DNA polymerase I?
5’ -> 3’ exonuclease which allows polymerase to move forwards removing nucleotides while also adding nucleotides
