Law: DNA mutations

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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
ie) carcinogens


1. chemical agents that can lead to cancer
2. any mutagen classified as potential for cancer can be considered a carcinogen.

Mutation Rate

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.

Point Mutations

1. change of 1 or 2 nucleotides in the DNA of a gene
2. may be bp substitutions or frameshift mutations

Base Pair Substitutions

1. nucleotide substitution replaces one nucleotide for another.

Silent Mutations

1. nucleotide change, but no amino acid change in protein

Nonsense 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)

Missense Mutations

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. DNA sequence change from GAG → AAG
2. AA exchange from Glu → Lys
3. Chromosome #6

Frameshift mutations

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

Ionizing Radiation

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. 1/1000rem

Radiation sickness

100 rem

Base Analogs

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

Base Modifiers

1. changes 1 base for another
2. chemicals that interact with bases
ie) HNO₂→C→U→U/A→A/T

DNA binders

1. intercalating agents
2. chemicals that resemble bp inserts into DNA
3. distorts the helix
4. leads to deletions or insertions
ie) ethidium bromide

Transition Mutation

1. can be purine to purine or pyrimidine to pyrimidine

Trasversion Mutation

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

Thymine Dimers

1. thymine bases adjacent to each other in same DNA strand pair with each other.

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

Xeroderma Pigmentosa

1. UV excision repair is effected - any of 8 different genes may be mutated
2. extreme sunlight sensitivity, skin lesions on exposure, and ↑ risk of skin cancer
3. auto recessive - 1/250000

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