DNA Mutations and Base Repair Mechanisms

Mutations

changes in DNA base sequence

Loss-of-function mutation
Gain-of-function mutation
Germline mutation
Somatic mutation
Conditional mutation
Reverse mutation
Splice mutation

DNA Mutations (8)

Base substitutions
Base insertion and deletion

Gene Mutations (2)

Transition
Transversion

Base substitutions

Frameshift mutation

Base insertion and deletion

Transition
Base Substitutions

purine-purine substitution or pyrimidine-pyrimidine substitution

Transversion
Base Substitutions

purine-pyrimidine substitution or vice versa

Thymine-guanine wobble

Cytosine-adenine protonated wobble

Base Substitutions

-Replication proceeds properly, resulting in one of the two new chromatids having the wrong base on both strands of its DNA
- During the next cell cycle, the strands with the mismatched bases serve as templates for DNA replication leading to incorporation of errors

strand slippage

Base Insertion and Deletion
Insertions and deletions may arise through _____________

Forward Mutation

Phenotypic Effects of Mutation
- wild-type to mutant

Reverse Mutation

Phenotypic Effects of Mutation
- mutant to wild-type

Missense

Phenotypic Effects of Mutation
- one Amino Acid to a different Amino Acid

Nonsense

Phenotypic Effects of Mutation
- Sense codon to a Nonsense (Stop) codon

Silent

Phenotypic Effects of Mutation
- Codon to a synonymous Codon (no change in Amino Acid)

Neutral

Phenotypic Effects of Mutation
- one Amino Acid to a new similar Amino Acid (no change in function)

Sickle cell anemia

Physical Example
– change in amino acid sequence causes the hemoglobin to crystalize at low O₂ levels.

Spontaneous Modification

Base modifications caused by hydrolytic cleavage and other chemical reactions

Hydrolysis reactions
A

Spontaneous Modification
1. __________ remove purine (A and G) rings by cleaving the N-C glycosidic bond that holds them to the sugar

2. An _-containing nucleotide is usually incorporated across from the depurinated one during the next round of replication

Pyrimidines

__________ can be deaminated by hydrolysis of the NH2 group without requiring an enzyme

Mutagens
Base analogs

Induced Mutations

Mutagens

physical or chemical agents that causes mutation

Base analogs

chemical that can substitute for a normal base in the DNA.

5-bromouracil

Base analogs
- ____________ in the enol form pairs with G. G → C a transition mutation

Alkylating agents
Deaminating agents
Hydroxylaminating agents

Mutagens
- _________________, _____________, _____________ chemically change functional groups on the DNA bases

Alkylating agents
Deaminating agents
Hydroxylaminating agents
Intercalators
Reactive-oxygen species
UV light radiation

Mutagens

Intercalators

flat hydrophobic, usually aromatic molecules that insert between stacked base pairs in DNA

Reactive-oxygen species

free radicals

UV light radiation

brings the two neighboring nucleotides closer together, leaving the bases unable to bond with the bases from the other DNA strand

Repair Mechanisms

• DNA polymerase proofreads itself, but makes an error every approximately 10,000-100,000 nucleotides (104 – 105)

• After repair, the frequency of errant nucleotides is only approximately 1 in 1 billion (109) nucleotides

Base mismatch
Direct
Base excision
Nucleotide excision

Common repair mechanisms

Base mismatch repair

Mismatched bases and loops such as

those that lead to deletions and

duplications form bubbles in the DNA

double helix, which are recognized by

the repair systems

O⁶-methylguanine DNA methyltransferase
photolyase

• Alkylation can be reversed by_______________________

• In bacteria, _____breaks the bonds that

maintain pyrimidine dimers

• Thymine dimers can be repaired by photo-

reactivating enzymes

Base Excision Repair

• In eukaryotes, DNA poly B which has no proofreading ability, fills in the gap

• DNA poly B’s error rate is high enough to leave  10 new mutations uncorrected per day

• The AP endonuclease undertakes the proofreading

Nucleotide Excision Repair

Acts on pyrimidine dimers and large

distortions of the DNA helix

Antimutagens

decrease the frequency of mutations and inhibit the mutagenic effect of chemical or physical agents