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Mutations, Mutagenesis, and Repair. Chapter 10 . The Problem. DNA extremely long, fragile Subject to both physical and chemical damage Consequences could be lethal for organism or offspring. }. Frameshift mutations. Mutation. A heritable change in the base sequence of DNA

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

}

Frameshift mutations

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
consequences of mutation
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 mutations—additions or deletions. Peptide may be longer or shorter.
  • Sense mutation?
other terms
Other Terms
  • Conditional Mutation—wild 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 doesn’t eliminate protein function
mutagenesis
Mutagenesis
  • The process of mutation
  • Mutagen—anything that promotes ort causes mutations
    • Chemical
    • Physical
mutation causes
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
  • Crosslinking—covalent linkage between bases
spontaneous mutations
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.
spontaneous mutations1

}

Altered H-bonding

Spontaneous Mutations
  • Tautomeric shifts during replication.
  • Depurination—if a purine base is lost from C-1 of deoxyribose, will get apurinic site.
    • Odds of misincorporation on the daughter strand=75%
    • Enzymes specific for this type of mutation have evolved
  • Deamination.
    • CU
    • AHypoxanthine
tautomers and mutation
Tautomers and Mutation

Normal base pairing

Rare imino forms of adenine and cytosine

Rare enol forms of thymine and guanine

Back

deamination of c and a
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
removing and replacing uracil
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)
base excision repair ber

G

G

P

P

P

P

P

P

P

P

P

P

P

P

P

P

P

P

T

C

G

T

U

C

G

A

G

C

A

G

G

C

A

G

C

A

C

P

P

P

P

P

P

P

P

P

P

P

P

P

P

Base Excision Repair (BER)

Uracil DNA glycosylase

AP endonuclease

G

DNA polymerase I

G

T

C

C

G

DNA ligase

T

G

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

?

deamination of cytosine and 5 methylcytosine
Deamination of Cytosine and 5-methylcytosine

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induced mutations
Induced Mutations
  • Caused by exposure to a mutagen
  • Causes
    • Exposure to base analogs
    • Chemical mutagens
    • Intercalating agents
    • Uv- radiation
    • Transposable elements
    • Mutator genes
exposure to bases analogs
Exposure to Bases Analogs
  • Base analogs—substances that are similar to and can substitute for standard bases
  • Examples—AZT, 5-bromouracil (5-BU) and 2-aminopurine (2-AP)
5 bromouracil
5 Bromouracil

THE PROCESS

A·T

  • 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: A·T G·C

Replication in presence of BrU

A·BrU

Tautomeric shift

A·BrU*

Replication

A·T + G·BrU*

Replication

A·T + A·T + G·BrU* + GC