geneAnatAndFunct - Anatomy and Function of a Gene:...

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Anatomy and Function of a Gene: Dissection through Mutations Chapter 7
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Outline What are mutations? - Types of mutations - How often mutations occur - What events cause mutations - How mutations affect survival and evolution What mutations tell us about gene? - Gene is further indivisible unit of function - Gene is a continuous region of DNA What is function of a gene? - “One gene – one enzyme – one reaction”: genes encode proteins or functional RNAs - Mutations that alter genes’ instructions for amino acids alter protein structure and function
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Mutations: Primary tools of genetic analysis Mutations are heritable changes that modify the information content of DNA CLASSIFICATION OF MUTATIONS BY NATURE OF A PHENOTYPIC CHANGE Forward mutation – changes wild-type to different allele Reverse mutation – causes novel mutation to revert back to wild- type (reversion)
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Classification of mutations by level of genome alteration Point mutations Chromosomal rearrangements Classification of mutations by effect on DNA molecule: Point mutations (within one gene): Substitution – base is replaced by one of the other three bases Deletion – block of one or more base pairs is lost Insertion – block of one or more base pairs is added Inversion - 180 o rotation of several or more base pairs Chromosomal rearrangements (usually affect more than one gene): Deletion – piece of DNA is lost Insertion – piece of DNA is added Inversion - 180 o rotation of piece of DNA Reciprocal translocation – parts of nonhomologous chromosomes change places
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Fig. 7.2 Transversions Substitutions Transitions A G T C Pur Pyr
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MUTATIONS (by origin) Spontaneous Induced
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Spontaneous mutations influencing phenotype occur at a very low rate Mutation rates from wild-type to recessive mutant alleles for five coat color genes in mice Fig. 7.3 b
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General observations of mutation rates Mutations affecting phenotype occur very rarely Different genes mutate at different rates Rate of forward mutation is almost always higher than rate of reverse mutation
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Are most mutations spontaneous or induced? Luria and Delbruck experiment (fluctuation test): A simple way to tell whether mutations arise spontaneously or are induced by a mutagenic agent In this experiment, mutant phenotype – resistance to antibiotic or other bactericide
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Fig. 7.4 
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Replica plating verifies preexisting mutations Fig. 7.5 a Phenotype – resistance to antibiotics (same approach could be used for auxotrophy)
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Fig.  7.5b
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Interpretation of Luria-Delbruck fluctuation test experiment and replica plating Bacterial resistance arises from mutations that exist before exposure to bacteriocide After exposure to bacteriocide, the bacteriocide becomes a selective agent killing the nonresistant cells, allowing only the preexisting resistant cells to survive Mutations do not arise in particular genes as a direct response to environmental change Mutations occur randomly at any time
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This note was uploaded on 12/17/2009 for the course BIO 2354 taught by Professor Brockett during the Spring '08 term at Georgia Institute of Technology.

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geneAnatAndFunct - Anatomy and Function of a Gene:...

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