DNA Replication Flashcards
What are the common features to all DNA polymerases?
- Template Dependent
- Primer dependent
- Catalyze addition of dNTP to 3’ endof growing chain
- Highly Accurate (misincorporate at ~10-6)
What is the mechanism of addition of a nucleotide to a DNA chain?
What is an exonuclease?
An enzyme that can remove a nucleotide from the end of a DNA chain (can be eithe 3’ or ‘5)
What is an endonuclease?
An enzyme that can cleave a nucleotide from within the chain (not just at 5’ or 3’end)
In which direction does DNA replication proceed?
It is bidirectional and both replication forks are synthesized simltaneously.
What are Okazaki Fragments?
Segments that are synthesized discontinuously on the lagging strand during DNA replication.
How are RNA primers removed from the end of newly synthesized DNA?
5’ Exonuclease.
How is the gap left by removal of RNA polymerase filled?
DNA polymerase
What seals/joins ends of Okazaki fragments?
DNA Ligase - reforms phosphodiester linkage between free 5’ phosphate group and free 3’ OH group
What are the names, functions, and exonuclase activity of the DNA ploymerases in E. coli?
What is the meant by the processivity of a DNA polymerase enzyme?
How many nucleotides can be added to the strand befor the enzyme falls off.
What is the function of a helicase?
To unwind the 2 strands of DNA in replication
How does a helicase unwind the double strande DNA molecule?
By hydrolysis of ATP
What is the purpose of single stranded binding proteins (SSB)?
To prevet the DNA strands from reannealing before it is replicated
What is the purpose of topoisomerases?
Allow unwinding to occur by relieving tension on the DNA strands by nicking and rejoining them as replication proceeds
During what phase of the cell cycle does DNA replication occur?
During S phase
What is 3’ overhang and what causes it? Why could it be a problem?
3’ overhang is the extra bases at the 3’ end of replicated DNA caused by removal of RNA primers at the 5’ end.
It can cause progressive shortening of replicated DNA, and potential loss of genetic information
What is the solution for 3’ overhang?
How does it work?
Telomeres . Synthesized by telomerase.
It is a tandemly repeated sequence on the end of chromosomes (AGGGTT in humans)
What is the problem caused by depurination?
Mutations occur (10,000 purines lost per day in mamalian genome)
What is a spontaneous mutation that can occur with cytosine?
Deamination to uracil. Approximately 100 times per day in mamalian genome
What mutation can be caused by UV light
Pyrimidine Dimer Formation
What is mismatch repair?
Repairs mistake in daughter strand - DNA segment replaced
What occurs in base excision repair?
Glycosylases recognizes Uracil, Oxidised Bases, Alkylated base, or pyrimidine dimers.
-AP endonuclease cleaves glycoside bond —->DNA polymerase repairs strand —-> DNA ligase rejoins backbone
Nucleotide Excision Repair
Recognize DNA damage e.g. Helix distortion
Damage modification
Damage removal
Replacement
