Gene Mutation and DNA Repair

Gene Mutation, DNA Repair, and Transposition

• Topic: Gene mutation, DNA repair, and transposition were discussed.
• Central Dogma of Molecular Biology: DNA undergoes transcription to mRNA, which then undergoes translation by ribosomes to create proteins within cells.

Gene Mutation Classification

• Mutation: Any base pair change in DNA alters the nucleotide sequence of an organism's genome. This can occur in somatic cells or germ cells, influencing genetic diversity and evolution.

Classification Based on Molecular Change

• Point Mutation (or Base Substitution): A change of one base pair to another in a DNA molecule.
  • Missense Mutation: A nucleotide change within a protein-coding triplet, resulting in a different amino acid.
  • Nonsense Mutation: A triplet is changed into a stop codon, leading to protein termination.
  • Silent Mutation: A point mutation alters a codon but does not affect the amino acid at that position.
  • Neutral Mutation: Mutation occurring in non-coding DNA, with no effect on gene products or expression.

Other Types of Point Mutation

• Transition: A pyrimidine replaces a pyrimidine or a purine replaces a purine.
• Transversion: A purine replaces a pyrimidine or vice versa.

Classification Based on Molecular Change: Frameshift Mutation

• Frameshift Mutation: Insertion or deletion of one or more nucleotides within a gene.
  • Result: Change in subsequent three-letter codons. May lead to a stop codon, halting polypeptide synthesis.

Classification Based on Effect on Function

• Loss-of-Function Mutation: Reduces or eliminates the function of the gene product.
• Null Mutation: Complete loss of function.
• Gain-of-Function Mutation: Codes for a gene product with enhanced or novel functions.
• Suppressor Mutation: A second mutation that reverses or relieves the effects of a previous mutation.
  • Intragenic: Suppressor mutation occurs within the same gene.
  • Intergenic: Suppressor mutation occurs in a different gene.

Classification Based on Phenotype

• Phenotypic effect classifications: Visible, Nutritional, Biochemical, Behavioral, Regulatory, Lethal, and Conditional.
• Examples provided for each classification (ranging from Mendel's peas to specific human diseases and animal traits).
• Example of conditional mutation: Shown in Himalayan rabbits.

Mutations Occur Spontaneously and Randomly

• Spontaneous Mutation: Changes in nucleotide sequence occurring naturally, not associated with specific agents.
  • Example: DNA replication.

Mutation Rate

• Rate of spontaneous mutation is typically low but varies between organisms and even between genes within the same species.

Spontaneous Mutations Arise from Replication Errors

• DNA Replication Errors: DNA polymerase may insert incorrect nucleotides during replication. It has 3’ to 5’ exonuclease activity which allows for proofreading.
• Tautomers: Bases that increase the chance of mispairing during DNA replication.

Spontaneous Mutations (Examples)

• Depurination: Loss of a nitrogenous base in DNA, commonly guanine or adenine.
• Deamination: Conversion of an amino group to a keto group, with cytosine converted to uracil and adenine to hypoxanthine.
• Oxidative Damage: Reactive oxygen species (free radicals) that create DNA chemical modifications and sometimes damage.
• Jumping Genes (Transposable Elements): These elements can cause mutations when moving within a genome.

Induced Mutations

• Induced Mutation: Resulting from exogenous (external) factors such as UV radiation (example given of skin cancer).

Examples of Induced Mutations

• Base Analogs
• Alkylating Agents
• Intercalating Agents
• Adduct-Forming Agents
• Ultraviolet (UV) Rays
• Ionizing Radiation

Single-Gene Mutation and Human Diseases

• Monogenic Diseases: Diseases caused by mutations in a single gene.
• OMIM (Online Mendelian Inheritance in Man): A database of human genes, mutations, and disorders.

Single-Gene Mutation and Diseases: Point Mutations

• Point Mutations: Mutations involving specific nucleotide changes, including missense and nonsense mutations
• Missense; results in aa substitution.
• Nonsense; a stop codon substitutes for an aa.
• Frameshift; Involves deletions or insertions of nucleotides that alter the sequence's reading frame.

Single-Gene Mutation and Diseases: Other Types

• Beta-thalassemia: caused by mutations in the beta-globin gene and can result in a decrease in red blood cells and oxygen delivery to the body.

Expandable DNA Repeats

• Expansion of trinucleotide repeats in some mutant genes.
• Disorders linked to trinucleotide repeat expansion: Fragile X syndrome, Myotonic dystrophy, and Huntington's disease
• Trinucleotide Repeat Expansios: Causes conditions like intellectual disability and muscle/neurological problems.

DNA Repair Systems

• Living systems have evolved sophisticated repair systems to counteract spontaneous and induced DNA damage, ensuring genetic integrity.
• Mentioned scientists who received Nobel Prizes for their work on DNA replication and repair.

Proofreading and Mismatch Repair

• DNA polymerase's proofreading capability (3' to 5' exonuclease) catches many errors.
• Mismatch Repair (MMR) mechanism deals with errors that occur after proofreading.

Postreplication Repair

• Repair system for DNA damage that has escaped earlier repair steps.
• Role of RecA protein: Directs recombinational exchange to repair gaps.

Photoreactivation Repair

• UV-induced damage can be repaired by visible light using photoreactivation enzymes.
• Thymine dimers: Cross-links caused by UV radiation, disrupted by photoreactivation enzymes which require activation by blue light.

Base and Nucleotide Excision Repair

• DNA repair system to correct damaged or inappropriate bases, involving enzymes to recognize damage sites and remove them.

Xeroderma Pigmentosa (XP)

• Recessive genetic disorder affecting NER pathways. Increased sensitivity to UV radiation; often leading to skin cancer.

Ames Test

• Assay used to determine a chemical compound's mutagenicity. It measures a compound's ability to cause mutations in bacteria.

Transposable Elements (TEs)

• DNA sequences that move within or between chromosomes.
• Types of transposable elements: DNA transposons and retrotransposons.
• Potential to cause mutations if the movement disrupts genes.

Retrotransposons

• A type of TE that expands itself by copying its contents via RNA intermediate, and then inserting that RNA back into the genome as DNA.
• These are not infectious as they don't have other necessary structural components like a retrovirus.

End of Lecture