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BIO207H5 (42)
Chapter 7

Chapter 7 - Textbook Notes.docx

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University of Toronto Mississauga
Nicole Myers

Chapter7DNAMutationDNARepairandTransposableElementsChromosomal mutationschanges involving whole chromosomes or sections of them Point mutationa change of one or a few base pairsmay change the phenotype of the organism if it occurs within the coding region of a gene or in the sequences regulating the geneGene mutationsaffect gene function can alter a phenotype by changing the function of the proteinTransposable elementsgenetic change occurring when certain mobile genetic elements in chromosome move from one location to another in the genome Transpositionchange in position DNA MutationAdaptation versus MutationAdaptationthe environment induced an adaptive inheritable changeSome geneticists thought that variation among organisms resulted from random mutations that sometimes happened to be adaptiveOthers thought that variations resulted from adaptation that is the environment induced an adaptive inheritable changeThe Adaptation TheoryLamarckismthe doctrine of the inheritance of acquired characteristics Keynote heritable adaptive traits result from random mutation rather than by adaptation as a result of induction by environmental influencesMutations DefinedMutationis the process by which the sequence of base pairs in a DNA molecule is alteredmay result in a change to either a DNA base pair or a chromosomeSomatic mutationin multicellular organisms the mutant characteristic affects only the individual in which the mutation occurs and it not passed on to the succeeding generationGermline mutationmay be transmitted by the gametes to the next generation producing an individual with the mutation in both its somatic and its germline cellsMutation rateprobability of a particular kind of mutation as a function of time such as the number of mutations per nucleotidepair per generationMutation frequencythe number of occurrences of a particular kind of mutation expressed as the proportion of cells or individuals in a population such as the number of mutations per 100000 organismsTypes of Point MutationsBasepair substitutionsa change from one base pair to another in DNA Depending on how the gene is translated to amino acid sequences it can result in no change to the protein an insignificant change or a noticeable change Two types1 a transition mutationfrom one purinepyrimidine base pair to the other purinepyrimidine base pair such as AT to GC this means that the purine on one strand of the DNA is changed to the other purine while the pyrimidine ion the complementary strand is changed to the other pyrimidine 2 a transversion mutationa mutation from a purinepyrimidine base pair to a pyrimidinepurine base pair such as GC to CG or AT to CG Missense mutationis a gene mutation in which a basepair change causes a change in an mRNA codon so that a different amino acid is inserted into the polypeptidea phenotypic change may or may not result depending on the amino acid change involvedNonsense mutationis a gene mutation in which a basepair change alters an mRNA codon for an amino acid to a stop nonsense codon UAG UGA UAAthis causes premature termination of polypeptide synthesis so shorter than normal polypeptide fragments often nonfunctional are released from the ribosomesNeutral mutationis a base pair change in a gene that changes a codon in the mRNA such that the resulting amino acid substitution produces no detectable change in the function of the protein translated from that messagea neutral mutation is a subset of missense mutations in which the new codon codes for a different amino acid that is chemically equivalent to the original or the amino acid is not functionally important and therefore does not affect the proteins function Silent mutation also known as a synonymous mutationis a mutation that changes a base pair in a gene but the altered codon in the mRNA specifies the same amino acid in the proteinin this case the protein obviously has wild type functionsilent mutations most often occur by changes such as this at the thirdwobbleposition of a codonFrameshift mutationusually results in a nonfunctional proteinif one or more base pairs are added or deleted from a proteincoding gene the reading frame of mRNA can change downstream of the mutationtherefore incorrect amino acids are added to the polypeptide chain after the mutationthese mutations may generate new stop codons resulting in a shortened or lengthened polypeptide chains
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