Module 8: Mutations and Genetic Recombination

Mutations and Genetic Recombination

Introduction to Mutations

• Mutations and mutants

  • Mutations are heritable changes in DNA leading to changes in organism properties.

  • Mutant is a form of a gene with a mutation, while wild type is the normal form.

  • Types of mutations include insertions, deletions, inversions, translocations, and base changes.

• Molecular basis of mutations

  • Induced mutations are caused by mutagens like chemicals or radiation.

  • Spontaneous mutations occur naturally during DNA synthesis.

Types of Mutations

• Point mutation

  • Involves a change in a single base pair sequence in DNA.

  • Can be silent, nonsense, or missense mutations.

• Insertion/Deletion

  • Frameshift mutation shifts the reading frame, while non-frameshift mutation doesn't.

Mutagenesis

• Chemical mutagens and radiation

  • Nucleotide base analogs and DNA-modifying agents induce mutations.

  • Alkylating agents chemically alter base structures leading to mutations.

Conclusion

• Mutations can be induced by various factors and result in different types of genetic changes.

• Understanding mutations is crucial in studying genetic recombination and variability in organisms.

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Mutagens and Genetic Recombination

• Alkylating agents can affect non-replicating DNA.

  • Base analogs and alkylating agents induce base-pair substitutions.

• Intercalating agents insert between DNA bases.

  • Induce insertions or deletions.

  • Acridines typically cause frameshift mutations.

  • Example: Ethidium bromide is an intercalating agent used in gel electrophoresis.

• Non-ionizing and ionizing radiation are highly mutagenic.

  • UV radiation leads to strand breakage and thymidine dimers.

  • Ionizing radiation causes double-stranded and single-stranded breaks.

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Types of Mutations

• Mutations are noticeable if they cause phenotypic changes.

• Wild-type refers to no mutations in a gene.

• Forward mutation changes wild-type to mutant.

• Reverse mutation restores wild-type phenotype.

• Suppressor mutation replaces a codon with a similar amino acid.

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Types of Mutants Continued

• Base-Pair Substitutions:

  • UAU codon results in a silent mutation.

  • UAG codon leads to a nonsense mutation.

  • AAC codon causes a missense mutation.

• Frameshift mutations alter the polypeptide sequence.

  • Insertions and deletions change the primary sequence.

  • Large insertions due to errors in genetic recombination.

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Reversion and Suppressors

• Revertant restores the original phenotype.

  • Same-site and second-site revertants.

• Reversion is a mutation back to the wild-type genotype.

• Suppressor mutation restores the wild-type phenotype.

  • Mutation in a different gene with the same enzymatic function.

  • Suppressor tRNA mutations allow translation to continue.

Mutations and Genetic Recombination

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• Types of Mutants

  • Mutant gene leads to mutant protein, resulting in observable phenotypic differences.

  • Categories include morphology changes, lethal mutants, conditional mutants, resistance mutants, and biochemical mutants.

    • Auxotrophs are unable to grow without a specific molecule like histidine.

    • Prototrophs can grow in the presence of a molecule.

• Mutant Isolation

  • Selection involves growing mutants on a selective medium.

  • Replica plating helps identify mutants by their inability to grow under specific conditions.

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• DNA Repair

  • Various mechanisms like proofreading, direct repair, mismatch repair, and nucleotide excision repair help maintain DNA integrity.

  • Enzymes like photolyase, alkyltransferase, MutS, MutH, Uvr endonuclease, DNA polymerase, DNA ligase, and DNA glycosylase play crucial roles.

• SOS Response

  • Mechanism for DNA damage repair involving LexA protein, RecA enzyme, and over 50 genes.

  • Recombinase repair replaces damaged DNA strands.

  • Error-prone and error-free DNA repair proteins are activated in response to DNA damage.

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• Gene Transfer and Variability

  • Horizontal gene transfer allows genetic material exchange between organisms, promoting evolution and metabolic diversity.

  • Transfer mechanisms include transformation, conjugation, and transduction.

  • Homologous recombination leads to the exchange of DNA between genetic elements.

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• Fate of Donor DNA during Horizontal Gene Transfer

  • Donor DNA can integrate into the recipient cell's chromosome, producing stable recombinants.

  • The fate of donor DNA depends on its ability to self-replicate and potential restrictions like CRISPR/Cas degradation.

Mutations and Genetic Recombination

Homologous Recombination

• Recombines DNA segments by integrating new strands and repairing strand breaks.

  • Process involves an endonuclease cutting DNA, single-stranded binding proteins stabilizing the strand, base pairing displacing the recipient DNA strand, RecA protein catalyzing cross-strand exchange, and ligase enzymes joining fragments.

• Results in variation in DNA sequences due to homologous strands.

Mobile DNA: Transposable Elements

• Mobile DNA can travel and includes random bits of DNA or Phage DNA.

• Transposable elements are stretches of DNA that can move to another site.

  • Found inserted into plasmids, chromosomes, or viral genomes.

  • Move by transposition, important for genome rearrangement and genetic analysis.

• Two major types are insertion sequences (IS) and transposons, both carrying transposase genes and inverted terminal repeats.

Transposition

• Involves transposase creating double-strand breaks and inserting the IS transposable element into a specific region of the chromosome.

• Two mechanisms: conservative transposition (excised and reinserted) and replicative transposition (new copy produced and inserted).

• DNA transfer methods include transformation, conjugation, and transduction.

Conjugation

• Direct transfer of DNA between bacteria through cell-to-cell contact.

• Involves plasmids like F plasmid, which contains fertility factors for conjugation.

• Process includes building a sex pilus between donor and recipient cells, replicating the plasmid, and transferring mobile plasmids to the recipient cell.

Overall, mutations and genetic recombination play crucial roles in generating genetic diversity and facilitating DNA transfer among microbial populations.

Mutations and Genetic Recombination

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• Cytoplasmic bridge formation for transferring F plasmid from F+ to F- cell.

  • Recipient cell gains ability to build sex pili and becomes a donor cell.

• Transformation in prokaryotic cells:

  • Cells take in DNA fragments from outside.

  • Can be plasmids, transposons, or insertion sequences.

  • Integration transforms the cell genetically.

• Example: Pathogenic organisms mixed with harmless bacteria can transform the harmless into harmful.

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• Transformation Process:

  • Free DNA binds to recipient cell's DNA-binding protein.

  • DNA integrates into the chromosome via RecA-mediated homologous recombination.

• Naturally competent cells produce receptor proteins for DNA binding.

• Induced competence methods: electric shock, calcium chloride, electroporation.

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• Transduction with bacteriophages:

  • Bacteriophages infect bacteria.

  • Lytic cycle: Phage replicates and lyses the cell.

  • Lysogenic cycle: Phage DNA integrates into bacterial chromosome.

• Prophage integrates within bacterial chromosome during lysogenic cycle.

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• Transduction Processes:

  • Generalized Transduction: Phage includes host DNA in transducing particles.

  • Specialized Transduction: Involves metabolic genes, prophage incorporation.

• Recombination occurs at homologous sides or sequences during transduction.

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