Lab Genetics - Yeast Mutagenesis

Mutations

  • Definition of Mutations:

    • A mutation is defined as any change in the sequence of DNA.

    • These changes can be completely random events.

Understanding Mutations

  • Key Points:

    • Mutations lead to alterations in protein expression.

    • They can result from:

    • Damage to DNA that is not repaired.

    • Errors during DNA replication.

    • Insertion or deletion of DNA segments due to mobile genetic elements.

Base Pairs in DNA and RNA

  • DNA Base Pairs: A, T, C, G

  • RNA Base Pairs: A, U, C, G

  • Process Overview:

    • Transcription: The synthesis of RNA from DNA.

    • Translation: The synthesis of proteins from RNA.

  • Diagram of Flow:

    • DNA → mRNA → Protein.

Codon Structure

  • Triplet Codon Positions:

    • First Position (U):

    • Codon example: UCU --> phenylalanine (phe)

    • Second Position (C):

    • Amino Acid Codes include various combinations that result in specific amino acids being coded based on the codon sequence.

    • Stop Codons are denoted, indicating the end of protein synthesis.

Mutation Classification

  • Types of Molecular Change:

    1. Point Mutation:

    • A change of a single base pair to another.

    1. Frameshift Mutation:

    • Involves insertion or deletion of nucleotides within a gene, altering all subsequent three-letter codons.

Examples of Mutations

  • Point Mutation:

    • Example: "THE CAT SAW THE DOG"

    • Missense Mutation example: "THE BAT SAW THE DOG"

  • Frameshift Mutation:

    • Deletion Example: "THE ATS AWT HED OG" (Loss of 'C')

    • Insertion Example: "THE CMA TSA WTH EDO G" (Insertion of 'M').

Detailed Mutation Types

  • Point Mutations include:

    • Missense Mutation:

    • A nucleotide change leads to a different amino acid in the protein product.

    • Nonsense Mutation:

    • A change in the triplet to a stop codon, terminating translation prematurely.

    • Silent Mutation:

    • Alters the codon but does not change the amino acid sequence.

    • Transition:

    • A pyrimidine replaces a pyrimidine (C→T) or a purine replaces a purine (A→G).

    • Transversion:

    • A purine replaces a pyrimidine (T→G) or vice versa (A→C).

Effects of Mutations on DNA

  • Frame Shift Effects:

    • Insertion and deletion can greatly affect the protein synthesis, resulting in varying amino acid sequences and potentially non-functional proteins.

Visualization of Mutation Effects on DNA

  • Normal DNA Sequence:

    • Template: T A A A A TTCGTGCA

    • Mutated Sequence Examples showcase silent, nonsense, and frameshift mutations followed by resulting changes in protein synthesis.

Base Pair Substitutions

  • Transitions:

    • More likely to result in synonymous substitutions due to genetic code degeneracy.

  • Transversions:

    • More often lead to non-synonomous substitutions due to more drastic base changes.

Purines and Pyrimidines

  • Purines: A, G

  • Pyrimidines: C, T, U

  • Characteristics:

    • Purines have a fused ring structure (6+5 membered rings).

    • Pyrimidines consist of a single 6 membered ring.

Mutation Types by Location

  • Somatic Mutations:

    • Occur in any cell of the body except gametes.

  • Germ-Line Mutations:

    • Occur in gametes, affecting the entire organism and offspring.

Classification by Chromosomal Location

  • Autosomal Mutations:

    • Occur in genes on autosomes (non-sex chromosomes).

  • X-linked Mutations:

    • Occur within genes on the X chromosome.

Phenotypic Effects of Mutations

  • Loss of Function: Reduces gene functionality.

  • Null Mutations: Complete loss of function.

  • Gain of Function: Enhances or provides new functions to gene products.

  • Visible Mutations: Affect morphological traits.

  • Auxotrophic Mutant Example: Loss of ability to synthesize essential compounds; e.g., yeast with ADE2 knockout displays red color.

  • Conditional Mutant: Triggered under specific conditions; e.g., temperature-sensitive mutations.

Causative Agents of Mutations

  • Spontaneous Mutations: Result from natural changes in DNA.

  • Induced Mutations: Caused by environmental factors.

  • Mutagen: Any agent increasing mutation rates above spontaneous levels.

Types of Mutagens

  • Biological Mutagens: e.g. viruses that can cause mutations, such as influenza, herpes, polio, AIDS, and rabies viruses.

  • Chemical Mutagens: e.g., Ethidium Bromide, which intercalates DNA causing damage.

  • Physical Mutagens: e.g., ionizing radiation, non-ionizing radiation that can break DNA bonds and cause various forms of damage.

  • UV Light: Creates pyrimidine dimers impacting DNA replication and structure.

Mechanism of Pyrimidine Dimer Formation

  • Caused by UV light, leading to mutational errors by covalently bonding adjacent pyrimidines.

  • Photoreactivation: Mechanism by which DNA photolyase repairs these dimers in the presence of light, restoring normal function.