S1: DNA Synthesis Flashcards
How is DNA replication described as?
Semi-conservative
What is the replication origins?
Specific sites where DNA replication is initiated
What recognises the replication origins?
Initiation complexes
Why does DNA at the origin unwind to form a replication bubble?
Allow access to the replication machinery
What phase of the cell cycle does DNA synthesis occur in?
(S) phase
It involves complete unwinding of the parental DNA
How long is the bacterial cell cycle and phases does it include?
20-30 minutes
M phase- S Phase
How long is the mammalian cell cycle and phases does this include?
16-24 hours
M Phase - G1- S - G2
Why is the mammalian cell cycle longer than the bacterial cycle?
Human have more chromosomes
Compare the number of replication origins in bacteria and mammalian cells
Bacteria DNA only has a single replication origin
Eukaryotic cells DNA replication initiates at multiple replication origins
What is DNA polymerase?
This is the enzyme that synthesises DNA and cells have multiple DNA polymerases
Name the types of polymerases and function in eukaryotic cells
Alpha – replication Beta – replication Gamma – mitochondrion Delta – replication (causes elongation) Epsilon – replication (causes elongation)
List the key properties of DNA polymerases
- Acts in 5’ to 3’ directions
- Utilises AT and CG base pairing to synthesise new DNA strands
- It requires a DNA template = a DNA or RNA primer, the four dNTP and Mg2+ ions
- Proof reading function
What does dNTP stand for?
Deoxyribosenucleoside triphosphate
Why does DNA polymerase require a DNA template?
DNA polymerase requires an -OH on the 3’ carbon to start adding nucleotide (building blocks can only be added to the 3’ end).
What are the building blocks for DNA?
dNTPs
What is the significance of DNA polymerase acting in the 5’-3’ direction?
One DNA strand must be made discontinuously while the other is made continuously.
What is a replication fork?
A replication fork will appear at the replication origin.
The parental strands will seperate and they are antiparallel.
What is the leading strand?
On the 5’-3’ strand of DNA, DNA polymerase will cause a continuous strand to be made called the leading strand. This is because as DNA opens up, DNA polymerase can attach to the 3’ end of DNA and continue to synthesis the DNA strand.
What is the lagging stand?
In the 3’-5’ strand, DNA polymerase is working in the opposite direction to the unwinding. This means that behind the polymerase, the strand will be opening up while it moves forward. This is called the lagging stand and it forms okazaki fragments. This is because replication has to be reinitiated again and again.
What does DNA Helicase do?
Seperates the base pairs producing single strands
What does Topoisomerase do?
Goes in front of helicase and gets rid of the coils in DNA (as it gets supercoiled by gyrase)
What does DNA primase do?
Lays down some RNA (produces 3’-OH) so that DNA polymerase knows where to start replicating
What do DNA binding proteins do?
Stabilise the single stranded DNA stop it being reannealed with the other parent strand.
What do replicative DNA polymerases do?
Copies parental strand
What do repair DNA polymerases do?
Repair the fragments and takes out the RNA so it will a full DNA strand
What does DNA ligase do?
Splices the fragments together (Okazaki fragments)
What does DNA polymerase have that ensures DNA replication proceeds with high fidelity?
Proof reading and mismatch repair system
- They check for mistakes and if made will go back and change them
- This makes error rate very low
Give an example of a disease from inherited defects in mismatch repair genes
Colon Cancer
failure to correct DNA synthesis may lead to cancer
What are DNA replication inhibitors used for?
DNA replication inhibitors are importantantibacterial, antitumour(so used in chemotherapy against cancers) andantiviral agents.
