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Flashcards in Repair Mechanisms Deck (23):

What are intercalating agents?

Mutagens that modify DNA causing indels by mimicking base pairs and slipping between stacked bases causing single nucleotide pair insertions or deletions.


What are some examples of intercalating agents?


Acridine orange



What light causes DNA damage response (DDR)?

UV light


Which UV radiation can penetrate earth's atmosphere?

UV-A and UV-B


What types of DNA lesions does UV light trigger?

Cyclobutane pyrimidine dimers (CPDs)

6-4 photoproducts (6 - 4 PPs)


What do the 2 lesions created by UV light do to DNA?

Both distort DNA's structure and impede transcription and replication.


Which part of the DNA is most susceptible to damage?

Relatively flexible areas of DNA double helix. One hotspot is within the commonly mutated p53 gene.


What happens if p53 is overexpressed?

The cell cycle is halted to allow for repair of mutation in p53.


How do pyrimidine dimers cause issues?

Dimers distort DNA helix and can block replication.


How are pyrimidine dimers repaired?

Sensor activates repair mechanisms by recruiting multiple proteins.

Effector proteins return DNA to native sequence or introduce variation which may cause disease or it can be tolerated.


What happens when DNA replication is halted?

DNA polymerase stops and some error prone polymerases (ER polymerases) take over replication and these finish of replication and are more likely to form mutated DNA strands.


What test is used to test mutagenicity?

The Ames test for mutagenicity.

This test uses auxotrophic histidine mutants of salmonella to test for reversion to prototrophy in presence of chemical being tested.

Without histidine these salmonella can't grow but if the mutagen caused a mutation then they can survive.


Many mutagens or their metabolites are also:



How can the ames test be modified to test for metabolites that are potential mutagens?

Using ground liver from another mammal (eg rats)


What types of mutations are required for ames test?

In TA1538 and TA1535: Indel mutations for reversion to prototroph

TA100: Base substitution GC -> TA transversions


Which DNA polymerases are involved in DNA repair?

Some DNA pols have 5' - 3' polymerase activity and have some proofreading 3' - 5' exonuclease activity. These are γ, δ, and ε.

Other DNA pols do not have proofreading such as: DNA pol α (primase activity and used in DNA repair) and DNA pol β (has a role in excision repair)

Lots of DNA pols which are used in translesion synthesis allowing replication when DNA is damaged and other repair processes include zeta, eta, theta, iota, kappa, lambda, mu, sigma, phi, Rev1


When are most important repairs made?

During DNA replication, proofreading repairs DNA mismatches and decreases number of mutations.


What are the general pathways for DNA repair?

Most repair requires 2 DNA strands so that one can act as a template for synthesis of the other.

Redundancy, most DNA damage can be repaired in multiple ways.


What is homology dependent repair?

Some repair systems utilise the antiparallel complementarity of the DNA double helix to restore damaged segments to original sequence. Section of DNA is removed and replaced with newly synthesized complementary DNA then DNA ligase seals the gap in the sugar phosphate backbone


What is mismatch repair?

Mismatch repair systems recognise mismatched bases by detecting distortions in DNA usually during replication.

Parental strand sequence is used to correct newly synthesized strand error.

Incorrect nt is detected and excised as well as nearby nt. Then DNA polymerase is used to repair gap and then this is joined by DNA ligase.

Mismatch repair can also recognize small unpaired DNA loops caused by slippage during DNA replication.


What is base-excision repair?

AP endonuclease excises damaged base and then entire nt is replaced.

Eukaryotes replace excised DNA with DNA pol beta.

DNA ligases that join sugar phosphate backbone at the end require the correct orientation of nucleotides for it to finish the process.


What is the problem with using base-excision repair to fix errors in DNA replication? How is this problem countered?

Eukaryotes use DNA pol beta with no proof reading activity so errors can be introduced. This can be significant given the number of base-excision repairs occurring in a human cell.

DNA ligases cannot join the sugar phosphate backbone if there is a mismatch because orientation is important.


How does base-excision repair take place?

Each DNA glycosylase recognizes and removes specific damaged bases producing apurinic/apyrimidinic site

AP endonuclease cleaves phosphodiester bond on 5' side and removes deoxyribose sugar

DNA polymerase adds new nucleotides to the exposed 3'-OH group

Nick in the sugar-phosphate backbone is sealed with DNA ligase.