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mutationsdna - Mutations and DNA Repair Mutations Heritable...

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Mutations and DNA Repair: November 20, 2009 Mutations Heritable changes in the nucleotide sequence or chromosome Mutations may be: -Spontaneous as a result of errors in DNA replication or -Induced by exposure to radiation, chemicals, viruses, or other mutagenic agents Somatic vs. Germ-line mutations Somatic Mutations occur in non-reproductive cells Mutations are not generally passed on to offspring Mitosis of somatic cells will pass on mutation to daughter cells Since only some cells are affected, phenotype may be minor Spontaneous Mutation Rates Studies suggest that mutations are rare 1/1,000,000 copies of a human gene Impact on the population of mutation is less severe because: -Nature of genetic code -Recessive mutations are not expressed in the heterozygote -Lower reproductive success or early death associated with many mutations - If mutated cells have a selective advantage, they may increase in number: Cancer Germ-line Mutations occur in reproductive cells Mutations may be passed onto progeny If mutation affects one homologous chromosome, expect approx 1/2 offspring to inherit the mutation Progeny who inherit mutations will have mutation in ALL cells (somatic and germ-line) Two General Categories …affect chromosomes mutations (Chpt 9) change nucleotide sequence (relatively small region) Types of Gene Mutations Base Substitutions: Point mutation: Substitution of one base for another Will result in change of one codon May affect one amino acid change Base Substitutions: Two Classes Transversions vs. Transitions
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Fig.  11. 11 Inheritance of Fragile-X syndrome Fig. 11.12 Transmitter male Affected males Unaffected females Fi g.  1 1. 1 3 Insertions and Deletions: Removal or addition of nucleotides Multiples of 3 will be in-frame insertions or deletions Removal or addition of other combinations will lead to frameshift mutations and potentially will affect many amino acids Trinucleotide Repeats Class of mutations associated with a number of genetic disorders Caused by an expansion of nucleotide triplets Process is allelic expansion when the gene size is increased by an increase in trinucleotide repeats Fragile-X Syndrome Approximately 1% of males institutionalized males for mental retardation have fragile-X syndrome
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