Chapter 7 - Mutations

Nucleotide Mutation and Example

A single nucleotide change in a DNA template strand can lead to the production of abnormal proteins. Ex) Sickle Cell Anemia

Forward Mutation

A wild-type allele mutates to a different allele

Reverse Mutation

A mutant allele reverts to a wild-type allele

Substitution Point Mutation

Switching one base for another

Transition Substitution

Purine substituted for a Purine

Pyrimidine substituted for a Pyrimidine

Transverse Substitution

Purine substituted for a Pyrimidine

Pyrimidine substituted for a Purine

Purines

Adenine and Guanine

Pyrimidines

Thymine, Cytosine, and Uracil

Deletion/Insertion Point Mutation

Losing or gaining one or more nucleotide pairs

Spontaneous Mutation Rate

Probability of mutations per gene per gamete

Average 2-12 10^-6 mutations per gene per gamete

How often do humans acquire spontaneous mutations?

Humans have 28,000 genes → 2-12 10^-6mutations per gene per gamete x 28,000 genes = 0.056 to 0.336 mutations per gamete

Fluctuation Experiment

Luria and Delbrück (1943)

If exposure led to mutation, the mutants should be evenly distributed on plates

If mutation leads to exposure, the mutants would be unevenly distributed across the plates.

They observed that the mutants were spread unevenly; therefore, mutation led to exposure and is spontaneous.

Genetic Screen

You let everything grow and look for mutants

Genetic Selection

You only let mutants grow

2 examples of mutations from natural processes

1. X-rays: create double-stranded breaks that may be ligated back together incorrectly

2. Ultraviolet light: creates thymine-thymine dimers (lesions caused by 2 thymines linking) which disrupt replication.

Primary purpose of 3′ to 5′ exonuclease

To proofread and remove mismatched nucleotides from new DNA strands.

How often does DNA polymerase make mistakes?

<1 in every 10⁹ base pairs

Unstable Trinucleotide Repeats

Repeated nucleotide sequences cause DNA polymerase to lose its place during replication. More repeats = higher probability of contraction/expansion

Trinucleotide Repeat Expansion

DNA polymerase backtracks and re-replicated nucleotides

Trinucleotide Repeat Contraction

DNA polymerase jumps ahead and skips replication of some nucleotides

Example of Trinucleotide Repeat Expansion disease

Huntington’s Disease

• PolyQ disease (glutamine repeats)

• Gain-of-function → acquires new, abnormal activity

• Dominant disease with negative effect

Example of Trinucleotide Repeat Contraction disease

Fragile X Syndrome

• Non-PolyQ disease (not glutamine repeats; arginine repeats)

• Loss-of-function → loss of function (ability to produce protein)

• X-linked recessive disease

Mutagen

A physical or chemical agent that can cause mutations

When can spontaneous mutations occur?

DNA replication, recombination, or repair

Examples of Mutagens

Radiation (X-rays, UV light)

Chemicals (carcinogens, processed foods, cosmetics, cleaning products)

Infectious Agents (Viruses and Bacteria)

What can the examples of mutagens cause?

Nucleotide Substitutions

Intercalating Agents

A flat, ring-shaped molecule that slips in between stacked base pairs of the double helix and can cause insertions or deletions

Mutations that affect the cell cycle can lead to…

Cancer

Ames Test Purpose, Procedure, and Conclusion.

Used to identify potential carcinogens.

• Potential mutagen mixed with His- bacteria (mutated bacteria that cannot grow)

• # of His+ bacteria compared to the control group

• Mammalian metabolic processes can turn something nonhazardous into a mutagen (experiment used rat liver enzyme)

If significant growth is observed, then the bacterial colonies were reverted to His+ (reverse mutation)

If no significant growth was observed, the bacterial colonies were not reverted.

Homology Dependent Repair

A cellular mechanism that fixes double-strand DNA breaks using a matching/homologous sequence as the repair blueprint

Base Excision Repair

Removes a single base and repairs

Nucleotide Excision Repair

Removes large DNA damage (usually by UV light → thymine-thymine dimers)

2 Mechanisms of Double Strand Break Repair

Homologous Recombination and Non-Homologous End Joining (NHEJ)

Homologous Recombination (Double Strand Break Repair)

Exonuclease chews back the damaged strand; DNA polymerase fills the gap from double-strand breaks using a sister chromatid or homologous chromosome as a blueprint.

Non-Homologous End Joining (NHEJ)

Proteins bind to the exposed ends of the broken strands, and DNA ligase reattaches them together.

How do bacteria tell which strand is the parent strand?

Methylation → parent strand is methylated

Mismatch Repair

Fixes errors from DNA replications

Xeroderma pigmentosum

Recessive mutation inhibiting nucleotide excision repair

Thymine dimers from UV light can’t be repaired

Accumulations of the mutations (dimers) cause excessive freckling and eventual skin cancer

BRCA1 and BRCA2 Genes

Code for proteins that are important for double-stranded DNA break repairs

Loss of 1 or both increases the risk of certain cancers (breast, ovarian, male breast, prostate, and pancreatic)