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Biology 331: Chapter 15 Gene Mutation Introduction: Mutation: The inherent tendency of organisms to change from one hereditary state to another Gene Mutation (Point mutation): When a gene changes from one allele to another Chromosome Mutation: Segments of chromosomes, chromosomes or sets of chromosomes change Forward mutation: Change away from the wild type Back mutation (reverse): Change from mutant to wild type From DNA to phenotype: Point Mutation: Change of one base pair or a small number of adjacent base pairs Substitutions: A change in a given base pair Transition: category Purine to purine or pyrimidine to pyrimidine Adenine (A); Guanine (G) Purine Cytosine (C), Thymine T) Pyrimidine Transversion: A replacement of a base of one chemical category with a base of the other A replacement within the same chemical Purine to pyrimidine or pyrimidine to purine Transitions are more common than transversions An important distinction in population genetics/systematics Missense Mutation: A codon for one amino acid is replaced by another Synonymous substitution: Change of an amino acid for a chemically similar one The affect is less severe Nonsynonymous substitution: Change an amino acid for one of different properties More severe implications Null mutations: Non functional mutants Likely due to changes in the active site (or allosteric) Leaky (partly inactivated) mutants: Likely due to changes away from the active site Nonsense mutations: A codon for an amino acid is converted to a stop codon Typically causes a totally nonfunctional protein unless it is near the 3' end Silent Substitutions: Degenerate DNA codons Changes to third position in a codon Does not change the AA Typically silent...but there are exceptions Additions/deletions: Losing or gaining a base pair Frameshift mutations Changes the reading fame Will screw up all of the AAs from the point of the insertion/deletion Can extend beyond the protein in question Typically causes a loss of function mutation Regulatory Mutations: Mutations to regulatory binding sites: Outcome hard to predict Does it affect the binding of a regulatory protein? If it does how much? How many regulators are there? Mutation to regulatory proteins: Affect is somewhat more predictable However, there are still multiple control systems Assorted Mutations Somatic -vs- Germinal Mutations: Somatic Mutations: Mutations in cells outside of the germ line The cell becomes the progenitor of a series of related cells These mitotically formed offspring are "clones" These cells tend to stay together in the organism Form "mutant sectors" The earlier the mutation occurs the more widespread the affects Cancer...a special case Somatic Mutations Plants: Can somatic mutations be passed on? In animals no In plants somatic cells can develop into germinal cells Germinal mutation: Mutations in the germ line Passed on to the next generation Assumes the mutant gamete gets into a zygote "normal" individuals can have "mutant" sex cells X-linked hemophilia in the royal family Started in Queen Victoria's germ line or that of her parents Mutation outcomes Mutant Types: May be subtle or gross Morphological mutations: Outwardly visible Lethal mutations: Are....uh....lethal? Morphological mutations Conditional Mutations: Mutants that only appear in some environments Restrictive condition: The environment that causes expression Permissive condition: Environment that does not cause expression Biochemical mutations: Change in the biochemical function of a cell Essentially underlies the other categories The term is often used for microorganisms Prototrophic: can survive on a basic medium of salts and an energy source (minimal medium) Auxotrophic: Require an additional nutrient to survive Biochemical Mutations Loss/gain of function? Loss of function mutations: Gain of function mutations: Less common than loss Why? Occurrence of mutations: How common are they? Organisms are stable generation to generation Implications? How common are mutations? The range is huge 500 fold difference seen in corn genes Mutation rate: Mutation rate per unit time Typically per generation or per cell division 1 in 14 generations, 1 in 100 etc. Mutation frequency: Frequency of a mutatnt in a population of cells Could be single cells, gametes, spores etc. Would yield a fractional value 0.0056 etc. Frequency of Mutations Cancer: A genetic disease Due to a somatic mutation Cancer genes (oncogenes) Start out as proto-oncogenes Typically related to regulation of cell division Proto-oncogenes convert to oncogenes via mutation Results in uncontrolled cell growth and metastasis These mutations can be cause spontaneously or by environmental factors Proto-oncogenes Genes in the germ line Repair mechanisms Regulation mutations Mutagens/carcinogens: Increase the rate of mutation EM radiation Chemicals Cigarettes Preservative Herbicides etc. etc. Mutagens