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What is DNA mutation?
1. Heritable changes in genetic material
2. DNA sequences change
3. chromosomal alterations
Why isn't DNA repair 100% effective?
DNA repair is not 100% effective because it allows for natural selection and evolution of species.
Benefits of Mutation
1. leads to genetic variation within species
2. evolution needs variation - natural selection
Causes of Spontaneous Mutations
1. are a part of normal cellular processes
2. DNA replication errors - DNA repairs errors
3. Endogenous waste damage - biproducts of cells that can damage DNA.
4. Nucleotide conversion events
1. external influences
2. physical agents and chemical agents that damage DNA
1. chemical agents that can lead to cancer
2. any mutagen classified as potential for cancer can be considered a carcinogen.
1. average of 1 mutation/ 1 million bp
2. Rate varies from gene to gene
3. rate varies with exposure
Mutation rate depends on...
1. frequency at which heritable changes occur in DNA
2. rate at which mutations are detected and repaired in cells.
3. whether mutation results in recognizable phenotype
Criterion for estimating mutation rate of a gene from pedigree analysis
1. single gene trait
2. dominant gene
3. gene must be completely present
4. must be certainty of paternity
5. no environmental trigger
6. gene must be completely penetrant
Reasons for Different mutation rates
1. gene size - bigger the gene = more mutations = ↑ mutation rate
2. trinucleotide repeats - more or less repeats ↑ mutation rate
3. chemicals - may affect C/G nucleotides, C can be converted to U and T causing a mutation.
1. change of 1 or 2 nucleotides in the DNA of a gene
2. may be bp substitutions or frameshift mutations
1. nucleotide change
2. leads to premature stop codon
3. sequence can be truncated or reduced
4. Functional and Non-functional protein will result - depends on location of mutation. Early mutations = non-functional proteins and Late mutations = dysfunctional proteins (sometimes)
1. nucleotide change
2. leads to AA change in translation
3. must have significant consequences
ie) sickle cell disease
4. may be neutral (no consequences)
1. DNA sequence change from GAG → GTG
2. AA exchange from Glu → Val
3. results in sickle cell disease
1. deletions and insertions
2. alters reading frame - very different, truncated (cause a stop codon to appear), run on (eliminate normal stop codon)
1. non-ionizing radiation
2. absorbed directly by DNA
3. Thymidine dimers occur - causes deletions during replication
ie) skin cancer is usually caused by over UV exposure
1. radiation that produces ions during interaction with other matter, including molecules in cells.
2. direct hit that causes deletions
3. indirect cause hits H₂O and forms free radicals by knocking H⁺ out of H₂O
1. radiation equivalent in man
2. amount of radiation that causes the same damage as a standard amount of xrays
3. mutation results from low dose exposure
1. mutagens that resemble nucleotides and are incorporated in DNA and RNA synthesis
2. may lead to wrong base pairings
ie) A/5-Bu (5-bromouracil) →5-Bu/G→G/C
1. changes 1 base for another
2. chemicals that interact with bases
1. intercalating agents
2. chemicals that resemble bp inserts into DNA
3. distorts the helix
4. leads to deletions or insertions
ie) ethidium bromide
1. purine to pyrimidine or vice versa
2. usually have more impact due to double ring structure changed to single ring structure or vice versa
DNA polymerase proof reading
1. has the ability to detect errors and move backwards to fix the problem.
2. occurs in S phase
DNA mismatch repair
1. mismatch bases missed by DNA Polymerase III
2. "Mut" proteins scan for these replication mismatches
3. scan after DNA polymerase III
4. DNA is cut out and fixed during S phase
DNA Excision Repair
1. existing DNA duplex is scanned for mutations (usually at G1/S checkpoint)
2. damage is cut out and repaired
3. many different repair enzymes
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