Chapter14_SSM - 65781_CH14_275_292.qxd 8/1/08 1:20 PM Page...

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275 Chapter 14: Molecular Mechanisms of Mutation and DNA Repair Chapter Summary Nucleotide substitution mutations may be transitions (interchange of A and G, or interchange of T and C) or transversions (any other change). Within a coding region, a missense (nonsynonymous) substitution results in an amino acid replacement, a synonymous (silent) substitution results in a synonymous codon, and a nonsense substitution results in a chain-terminating codon (UAA, UAG, or UGA). Insertions or deletions that are not exact multiples of three nucleotides result in frameshift mutations, in which the translational reading frame is offset from the point of mutation onward. Conditional mutations are expressed under one set of conditions (restrictive conditions, such as high temperature) and are not expressed under another set (permissive conditions, such as low temperature). In organisms in which cytosines may be methylated, the sites of methylation are often hot spots of mutation because deamination of 5-methyl cytosine produces thymine, which results in a C–G to T–A transition. In the human genome, some trinucleotide repeats are prone to expansion (increase in copy number), and the probability of expansion of the premutation state can approach 100 percent. Transposable elements are capable of moving from one location in the genome to another. The main classes of transposable elements are cut-and-paste elements, which typically have inverted repeats and transpose via a transposase; LTR retrotransposable elements, which have long direct or inverted repeats and transpose via a reverse transcriptase; and non-LTR retrotransposable elements, which also transpose via a reverse transcriptase but have no direct or inverted repeats. Insertions of transposable elements are the cause of many visible mutations. Some organisms have evolved mutational mechanisms for silencing repeated sequences—for example, RIP (repeat-induced point mutation) in Neurospora. Mutations can be induced chemically by direct alteration of DNA—for example, by nitrous acid, which deaminates bases. Base analogs, such as 5-bromouracil, are incorporated into DNA in replication. They undergo mispairing more often than the normal bases, giving rise to transition mutations. Alkylating agents react with bases and cause mispairing. Acridine molecules give rise to frameshift mutations, usually of one or two bases. Ionizing radiation results in oxidative free radicals that cause a variety of alterations in DNA,
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276 including single-stranded and double-stranded breaks. Although the amount of genetic damage from nor- mal background and other sources of radiation is believed to be low, studies of persons exposed to fallout from the Chernobyl nuclear meltdown indicate an increased mutation rate, at least for simple sequence repeat polymorphisms. A variety of systems exist for repairing damage to DNA. The mismatch repair system corrects mispaired
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Chapter14_SSM - 65781_CH14_275_292.qxd 8/1/08 1:20 PM Page...

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