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Microbiology: Changes in Genetic Material

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  • What is a mutation?
    A mutation is a permanent change in the base sequence of DNA that can alter the function of the encoded gene product.
  • What is natural selection in bacteria?
    The survival and reproduction of bacteria with new genotypes favored by environmental conditions is called natural selection.
  • What is a base substitution mutation?
    A base substitution or point mutation replaces a single base pair in DNA, potentially changing an amino acid in the protein.
  • Define missense mutation.
    A missense mutation is a base substitution that changes one amino acid in the protein sequence.
  • What is a nonsense mutation?
    A nonsense mutation creates a stop codon prematurely, resulting in a truncated, usually nonfunctional protein.
  • What is a frameshift mutation?
    A frameshift mutation involves insertion or deletion of nucleotides that shifts the reading frame, altering many downstream amino acids.
  • What causes spontaneous mutations?
    Spontaneous mutations occur due to occasional errors during DNA replication without external mutagenic agents.
  • What are mutagens?
    Mutagens are environmental agents like chemicals or radiation that increase the mutation rate by damaging DNA.
  • How does nitrous acid act as a mutagen?
    Nitrous acid alters adenine so it pairs with cytosine instead of thymine, causing base-pair substitutions during replication.
  • What are nucleoside analogs?
    Nucleoside analogs are chemical mutagens structurally similar to normal bases but cause incorrect base pairing when incorporated into DNA.
  • How does UV light cause mutations?
    UV light induces thymine dimers by covalently linking adjacent thymines, distorting DNA and blocking replication and transcription.
  • What is photoreactivation?
    Photoreactivation uses light-repair enzymes (photolyases) to reverse thymine dimers caused by UV light.
  • Describe nucleotide excision repair.
    Nucleotide excision repair removes damaged DNA segments and fills the gap with newly synthesized DNA complementary to the correct strand.
  • What is the typical spontaneous mutation rate?
    The spontaneous mutation rate is about \(10^{-6}\) mutations per replicated gene.
  • How do mutagens affect mutation rates?
    Mutagens increase mutation rates by 10 to 1000 times, raising the rate from \(10^{-6}\) to between \(10^{-5}\) and \(10^{-3}\).
  • What is positive (direct) selection of mutants?
    Positive selection identifies mutants by allowing only mutant cells with a specific trait (e.g., antibiotic resistance) to grow.
  • What is negative (indirect) selection using replica plating?
    Negative selection identifies mutants that cannot perform a function by comparing growth on media with and without a required nutrient.
  • Define auxotroph.
    An auxotroph is a mutant microorganism that requires a specific growth factor absent in the parent strain.
  • What is the Ames test used for?
    The Ames test detects mutagenic and potential carcinogenic chemicals by measuring the reversion of histidine auxotrophs to prototrophs in bacteria.
  • Why is rat liver extract used in the Ames test?
    Rat liver extract provides enzymes that activate chemicals into mutagenic forms, mimicking metabolism in animals.
  • Are all mutagens carcinogens?
    No, but about 90% of substances positive in the Ames test are also carcinogenic in animals.