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Chapter 15 - Gene Mutation, DNA Repair, and Transposition

Chapter 15 - Gene Mutation, DNA Repair, and Transposition -...

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Chapter 15 Gene Mutation, DNA Repair, and Transposition mutations classified in various ways - spontaneous, induced, and adaptive mutations o spontaneous mutations happen naturally no specific agents associated with occurrence, generally assumed to be random changes in the nucleotide sequences of genes often occur during enzymatic process of DNA replication o induced mutations mutations that result from influence of an extraneous factor may be result of either natural or artificial agents o adaptive mutations idea that organisms may ‘select’ or ‘direct’ the nature of gene mutation in order to adapt to a particular environmental pressure luria-delbruck fluctuation test presentation of first direct evidence that mutations are not adaptive, but occur spontaneously hypothesis 1 – adaptive mutation predicts that if a constant number of bacteria and phages is present on each plate, and if the incubation time is constant, there should be little fluctuation in the number of resistant cells from plate to plate and from experiment to experiment hypothesis 2 – spontaneous mutation predicts that the number of resistant cells will fluctuate significantly from experiment to experiment, and from tube to tube, reflecting the varying times at which most spontaneous mutations occurred in liquid culture - classification based on location of mutation – according to cell type or chromosomal locations o somatic mutations – may occur in any cell in the body except germ cells o germline mutations – occur in gametes o autosomal mutations – occur within genes located on the autosomes recessive autosomal mutations expression of most such mutations is likely to be masked by wild-type allele dominant autosomal mutations expressed phenotypically in the first generation o X-linked mutations – occur within genes located on the X chromosome X-linked recessive mutations arising in gametes of homogametic female may be expressed in hemizygous male offspring o haploinsufficiency remaining wild-type allele cannot synthesize sufficient levels of normal gene product to carry out the gene product’s function
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- classification based on type of molecular change – in terms of nucleotide changes that create mutation o point/base mutation change of one base pair to another in a DNA molecule missense mutation change of one nucleotide of a triplet within a protein-coding portion of a gene may result in the creation of a new triplet that codes for a different amino acid in the protein product nonsense mutation triplet will be changed into a stop codon, resulting in the termination of translation of the protein silent mutation point mutation alters codon but does not result in a change in the amino acid at that position in the protein o two other terms used to describe base substation transition pyrimidine replaces pyrimidine or a purine replaces a purine transversion
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