Mutations, Mutagenesis, and Repair

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Mutations, Mutagenesis, and Repair

• Chapter 10

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The Problem

• DNA extremely long, fragile
• Subject to both physical and chemical damage
• Consequences could be lethal for organism or
  offspring

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Mutation

• A heritable change in the base sequence of DNA
• Point mutation- change in a single base position
• Additions
• Deletions
• Substitutions
• Transitions
• Transversions
• Multiple mutations

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Consequences of Mutation

• Silent Mutation---base change, no amino acid
  change
• Neutral Mutation--- Base change resulting in aa
  change that does not affect protein function
• EX. Apartic acid (D) ?Glutamic acid (E)
• Missense mutation---altered codon, new aa with
  different chemical properties. Function affected.
  
• Nonsense mutation---base pair substitution
  results in a stop codon (and shorter polypeptide)
• Frameshift mutationsadditions or deletions.
  Peptide may be longer or shorter.
• Sense mutation?

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Other Terms

• Conditional Mutationwild type function except
  under certain (permissive) conditions
• Ex. Temperature sensitive mutants show mutant
  phenotype only at certain temperatures
• Leaky mutations a missense amino acid change
  that reduces but doesnt eliminate protein
  function

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Mutagenesis

• The process of mutation
• Mutagenanything that promotes ort causes
  mutations
• Chemical
• Physical

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Mutation-Causes

• Incorrect base pairing due to tautomeric shifts
• Removal of nitrogenous bases
• Alteration of nitrogenous bases
• Addition or deletion of nucleotides
• Single strand breaks
• Double strand breaks
• Crosslinkingcovalent linkage between bases

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Spontaneous Mutations

• Arise without mutagenic agents. DNA pol has
  proofreading function, can remove mismatched base
• Even if DNA pol misses a mismatch other systems
  can recognize and repair it.
• Recognition?
• Hemimethylation-allows enzymes to distinguish
  between parent and daughter strands.

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Spontaneous Mutations

• Tautomeric shifts during replication.
• Depurinationif a purine base is lost from C-1 of
  deoxyribose, will get apurinic site.
• ?Odds of misincorporation on the daughter
  strand75
• Enzymes specific for this type of mutation have
  evolved
• Deamination.
• C?U
• A?Hypoxanthine

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Tautomers and Mutation

• Normal base pairing

Rare imino forms of adenine and cytosine
Rare enol forms of thymine and guanine
Back
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Deamination of C and A

• C?U
• 3 H-bonds w/G?2 H-bonds w/A
• A?Hypo-xanthine
• 2 H-bonds w/G?3 H-bonds w/C

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Removing and Replacing Uracil

• Uracil automatically removed from DNA by uracil
  N-glycosylase
• AP Endonuclease cuts 5 to
  apurinic site
• Sugar phosphate removed
  by phosphodiesterase
• DNA pol I adds correct base
• Ligase seals
• Base Excision Repair (BER)

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Base Excision Repair (BER)
Uracil DNA glycosylase
AP endonuclease
G
DNA polymerase I
G
T
C
C
G
DNA ligase
T
G
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5 Methyl Cytosine Deamination

• Easily recognized and corrected
• What about 5-methyl cytosine?
• Is there a problem?
• Always remove T from a GT pair

?
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Deamination of Cytosine and 5-methylcytosine
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Induced Mutations

• Caused by exposure to a mutagen
• Causes
• Exposure to base analogs
• Chemical mutagens
• Intercalating agents
• Uv- radiation
• Transposable elements
• Mutator genes

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Exposure to Bases Analogs

• Base analogssubstances that are similar to and
  can substitute for standard bases
• ExamplesAZT, 5-bromouracil (5-BU) and
  2-aminopurine (2-AP)

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5 Bromouracil

• The Problem 5 bromouracil assumes the enol form
  at a much higher frequency than T
• ?if it replaces T, will probably get a mutation
  due to tautomerization during replication
• Result AT ?GC

THE PROCESS
AT
Replication in presence of BrU
ABrU
Tautomeric shift
ABrU
Replication
AT GBrU
Replication
AT AT GBrU GC