Likely result is polypeptide with no function

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Likely result is polypeptide with no function (nullimorph) or abnormal function (hypomorph). Change of stop codon to a sense codon Polypeptide has extra segment at the end. Possible result is polypeptide with no function, reduced function, or abnormal function (amorph, hypomorph, or neomorph). Nonsense mutations change a sense codon for an amino acid to a nonsense = stop codon. Shortened polypeptide which usually, but not always, has reduced or no function. Copyrighted figure removed.
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Mutations Due to Transposable Elements Transposable elements (TEs) are segments of DNA that can move from one location to another in the genome, or have a copy made and moved to a new location. Insertion of a TE in a gene can cause frameshifts or cause additional amino acids to be added to a protein product. This usually results in a null mutation. Insertion of TEs in controlling elements or between a gene and its controlling elements can cause major changes in transcription (no transcription or transcription at inappropriate times and/or places. Transposable elements are more common in some organisms (e.g. Drosophila melanogaster ) than in others and can cause a major proportion of all visible mutations.
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Genes Encoding Enzymes and Auxotrophic Mutants Some genes code for enzymes. Many enzymes catalyze steps in biosynthetic pathways. Steps are sequential, enzymes act sequentially. Usually, 1 enzyme catalyzes 1 reaction or step in a pathway. Genes G1 G2 G3 Enzymes E1 E2 E3 A B C D e.g. tryptophan biosynthesis in E. coli : Text says 5 steps but don’t show the pathway. Chorismic acid Anthranilic acid PRA CDRP InGP Indole L-Tryptophan ASase trpE PRTase trpD InGPSase trpC TSaseB trpB TSaseA trpA Notice that names of genes are italicized (or underlined ).
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Genes Encoding Enzymes and Auxotrophic Mutants Special classes of mutants especially useful with microorganisms: Antibiotic-resistant Auxotrophic: can’t make essential nutrient, must be supplied in culture medium Test for growth on minimal medium (MM) which has things wild type needs (energy, C, N sources; salts; etc.); only wild type (prototroph) grows, because it can make everything it needs from these simple ingredients. Control is complete medium (CM) which has yeast extract, proteose peptone, etc. that supply everything; all genotypes grow. One Gene - One Enzyme Hypothesis: not quite right, but focused on idea that genes have their phenotypic effects by encoding proteins.
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Tryptophan Auxotrophs Chorismic acid Anthranilic acid PRA CDRP InGP Indole L-Tryptophan ASase trpE PRTase trpD InGPSase trpC TSaseB trpB TSaseA trpA Grows On CM MM MM+Trp MM+indole MM+InGP MM+CDRP trp + + + + + + + trpA + - + - - - trpB + - + - - - trpC + - + + + - trpD + - + + + + trpE + - + + + +
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Visible Phenotypes of Pathway Mutants Drosophila melanogaster wild type has red eyes, which require a number of pigments. Makes eye pigment xanthommatin from tryptophan. cinnabar ( cn + ) gene encodes kynurenine-3-hydroxylase. cinnabar ( cn ) mutant has cinnabar-colored eyes. cinnabar
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Phenotypic Effects of Mutations in Introns Usually no effect unless mutation in splicing signal sequence --> failure to remove intron or splicing at incorrect site.
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  • Fall '19
  • Mutation, transposable elements, Leu

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