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23 Cards in this Set
- Front
- Back
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What are the 3 types of mutations?
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Substitutions, Deletions, Insertions
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What are the 2 types of Substitution?
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Transitions = Purine => Purine or Pyrimidine => Pyrimidine
Transversions = Purine => Pyrimidine or Pyrimidine => Purine |
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What is the most frequent type of mutation?
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Substitutions
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What is tautotmerization?
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A major cause of mutation that can occur in the absence of DNA damaging agents. Amino groups tautomerize to imino groups and keto groups tautomerize to enols. The tautomers then base pair inappropriately,
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What are the 6 agents that can damage DNA causing mutations?
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Base analogs
Bases Intercalating agents Acid/Heat Ionizing radiation Sunlight |
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What are two examples of a base analog?
How are they damaging? |
5-bromo-uracil and
2-aminopurine They are incorporated into DNA by the replication machinery and cause inappropriate base pairing. |
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How are bases damaging to DNA?
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They can be chemically modified such that they no longer base pair appropriately. Two chemicals that modify bases are nitrous acid and hydroxyl-amine.
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What are intercalating agents?
What is an example of one? |
Agents that slip in between the tacked base pairs. Intercalation-induced mutations generally involve deletion of one or a few bps.
Ethidium bromide |
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How are acid/heat damaging to DNA?
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They remove purines by breaking the nucleoside bond between the base and deoxyribose. This leaves bare apurinic sites.
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How is ionizing radiation damaging to DNA?
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They cause strand breaks and promote gross deletions and rearrangements.
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How is sunlight damaging to DNA?
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UV damage is done by linking thymine together yielding thymine dimers.
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T/F The pathways of DNA repair between E. coli and Humans is very similar
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True
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DNA repair generally proceed in 4 steps. What are they?
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1- Recognition of the damaged region by an endonuclease that makes an incision
2- An exonuclease removes DNA adjacent to the damaged base on the damaged strand only. 3- The gap is filled in by DNA polymerase 4- The nick is sealed by DNA ligase |
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Spontaneous deamination of Cytosine results in this.
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Formation of uracil creating a G-U pair. Replication of the U strand results in an A instead of a G in the daughter strand (Point Mutation)
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How is this repaired?
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By the Base Excision Pathway. Uracil is removed to generate a "toothless gap" in the DNA. AP endonuclease processes the gap, DNA polymerase I and DNA ligase complete the pathway using the undamaged parental strand as a template.
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How are thymine dimers repaired?
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By the nucleotide excision repair pathway
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Describe this process
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The dimers are excised by uvrABC exonuclease.
DNA Polymerase I then synthesizes a new strand where the gap has formed. The ends are joined by DNA Ligase |
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How are mismatched base pairs repaired?
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The mismatch repair pathway
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How does mismatch repair work?
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It occurs soon after DNA replication. DNA is methylated at specific sequences in the genome. An exonuclease removes new (unmethylated) DNA and removes the newly synthesized DNA. DNA polymerase III repolymerizes the strand correctly.
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An inherited susceptibility to cancer is usually caused by this.
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Defects in DNA repair enzymes.
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What causes Xeroderma pigmentosum?
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Malignant skin tumors resulting from an inability to repair thymine-thymine dimers
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What is Lynch Syndrome?
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A predisposition to colon cancer
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What causes Lynch Syndrome?
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A DNA repair deficiency. A gene named MSH2 is defective and is responsible for encoding an enzyme that is the human homolog of E. coli's MutS. MutS is responsible for repairing mismatches in DNA. (Mismatch Repair)
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