BSC 219 Lecture Notes - Lecture 6: Crosslinking Of Dna, Dna Repair, Dna Mismatch Repair

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7 Feb 2017
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Thursday, February 2, 2017
1
DNA Repair
Proofreading by DNA polymerase III
- DNA polymerase II makes/inserts the wrong base 1/100,000 times
- Proofreading catches these mistakes 99% of the time
- The overall error rate is 1/10 million bases
- This equals 300 mistakes every time the human cell replicates
DNA Repair
- Typically occurs during replication
- It is critically important for the cell to repair any DNA damage
- In adults, failure to properly repair the DNA can lead to:
Dysfunctional cell
Death of a cell
Cancer
- Different types of repair
During replication
- Repairing mismatches
- Repairing cross linking
- Repair double stranded breaks
Repair post replication
Mismatch Repair
- Repairs mismatched pairs that the DNA pol misses during replication
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Thursday, February 2, 2017
2
- Active during replication
- Key enzymes in E.Coli are MutS and MutL
- Homologous enzymes in humans are hMSH2 and hMLH1
- In E.coli, Adeline's are methylated on the template strand
When new strand is made it is not methylated to tell which is original
Allows the mismatch repair system to determine which base of the pair is incorrect
The DNA surrounding the incorrect base is removed and the new DNA is
synthesized
Repairing double stranded breaks
- Double stranded breaks occur when both strands of the DNA are cut
- Can occur spontaneously or by exposure to ionizing radiation
- 2 different ways to repair
Homologous-recombination based repair
Non-homologous end-joining (NHEJ) repair
- Homologous-recombination
The broken strand partners with the complementary strand on the sister chromatid
The heteroduplex is resolved so the 2 sister chromatids come back apart
The gaps are filed by DNA synthesis and ligation
- Nonhomologous end-joining
If the sister chromatids cannot be found, the loose ends of DNA must be attached
without guidance
This type of repair usually leaves behind mutations
The two broken ends are lighted (glued) back together
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