19 - DNA replication Flashcards
What is DNA replication described as?
Semi-conservative
What must happen in order for DNA to be replicated?
The 2 strands of the double helix must be separated, so each strand can be used as a template.
In what direction does DNA synthesis occur?
In a 5’ to 3’ direction with the primer strand always being antiparallel.
How is DNA synthesis initiated?
By creating a replication fork, where the DNA strands are separated.
Why can’t both strands of the DNA be synthesised continuously and how is this solved?
Due to the anti-parallel orientation of the strands the 5’-3’ direction, to solve this there are small fragments of DNA called Okazaki fragments on the lagging stand to allow replication to occur.
What are DNA polymerases responsible for?
Synthesising DNA chains.
Why are Primers important?
DNA polymerase can’t start making a DNA chain from scratch it needs the short chain of nucleotides which is the Primer.
What is the role of ribonuclease H?
To remove the primers form the DNA.
How are short RNA primers synthesised?
Using template and NTPs using DNA primase.
What does the lagging strand require to turn Okazaki fragments into a continuous strand of DNA?
DNA primase, DNA polymerase, Ribonuclease H and DNA ligase.
What is the role of DNA helicase?
To use ATP to separate parental DNA strands at the replication fork moving the fork forward.
What diseases can mutations in genes encoding helicase cause?
- Werner Syndrome - premature aging (autosomal recessive)
- Bloom syndrome - a rare cancer syndrome caused by the loss of function mutation.
What is processivity?
An enzymes ability to catalyse reactions without releasing its substrate
What enhances the processivity of DNA polymerase?
Their association with a sliding clamp.
What is a sliding clamp?
A structure that encircles DNA to help move the DNA polymerase forward.
What is the role of SSBs (Single stranded binding proteins)?
To keep the DNA fork open and enhance the processivity of DNA polymerase.
What is the role of topoisomerases?
- To prevent DNA form becoming tangled during DNA replication by releasing tension caused by the helicase unwinding. They do is by cutting and resealing the DNA.
- It also enhances the processivity of DNA polymerase.
What does chromosome shortening risk?
The loss of valuable coding info
What does the addition of TTAGGG repeats by telomerase do?
It compensates for the loss of telomere sequences caused by RNA primer removal preventing chromosome shortening.
