DNA Replication Flashcards
What are the DNA base pairs and how many bonds do they have btw them?
A and T (2x H bonds)
G and C (3x H bones)
What direction is DNA (or RNA) always synthesised in?
5’ to 3’ direction
Because the newly synthesised strand is running in the 5’ to 3’ direction, what direction is the template strand said to be running in?
The 3’ to 5’ direction
What is the first step of DNA replication?
The helicase enzyme comes along, recognising the region of replication (as the area’s with more A-T pairings as those are weaker and therefore easier to pull apart) and starts to separate the two DNA strands. This happens in multiple different places along the length of DNA (so lots of replication bubbles are formed)
How is the leading strand synthesised?
Continuously synthesised in the 5’ to 3’ direction (goes towards the replication fork)
How is the lagging strand synthesised?
Discontinuously synthesised in the 5’ to 3’ direction forming Okazaki fragments (goes away from its closest replication fork).
Therefore, DNA replication is semi-discontinuous.
What does the enzyme primase do?
It has its own internal 3’ -OH group which allows it to add nucleotides from scratch, creating an RNA primer (the starting point for DNA polymerisation).
On the leading strand it makes one RNA primer and that’s all that strand needs to make a continuous strand of DNA. On the lagging strand it has to continuously keep going back once more of the DNA is unravelled and make more RNA primers.
What does the enzyme DNA Polymerase III do?
It adds nucleotides to the RNA primer (because it has the -OH group that this enzyme needs to be able to start adding nucleotides), forming a strand of DNA.
What does the enzyme Ligase do?
It can form phosphodiester bonds if there is an -OH group and a Phosphate present. It goes along the short fragments (Okazaki) of DNA and bonds them together, creating one long strand of DNA.
It ALSO joins together the newly synthesised fragments from the multiple replication bubbles, including the leading strands.
What do Single Stranded DNA Binding Proteins do?
They keep the unravelled strands of DNA unravelled so they don’t snap back together before they have been used as template strands to make more DNA. They also protect the separated strands from being degraded.
What does the Topoisomerase enzyme do in DNA replication?
It moves ahead of the replication fork and cuts the DNA to allow it to unravel and release the tension the helicase enzyme has produced by unwinding the DNA strands. After the tension is released, topoisomerase glues the DNA back together again.
What does the enzyme DNA polymerase I do?
It has two functions, It recognises and removes/degrades RNA primers AND fills the gap with DNA nucleotides so that all the little fragments are end to end, ready for the ligase to come and make the bonds that will connect them
When can DNA errors be repaired and how?
During replication:
- Using EXOnuclease which removes nucleotides from the ends of the strands. Two names: the one that removes from the 5’ end is called 5’ to 3’ nuclease and the one that removes from the 3’ end is called 3’ to 5’ nuclease.
- Using ENDOnuclease which removes nucleotides from the inside of the strand
When DNA Polymerase III finds a mistake using its proofreading mechanism, how does it fix it?
The incorrect bases are removed using 3’ to 5’ EXOnuclease activity of DNA Pol III
What things can cause DNA damage after DNA replication?
- Incorrectly inserted bases are not corrected by DNA pol III
- Radiation damae
- Toxins (chemical damage)
