PCB3063'Exam'4'Chapter 14: Gene Mutation, DNA Repair and Transposition 14.1 • Mutations occur in regions that code for a protein or within noncoding regions like introns or regulatory sequences • May or may not affect phenotype • Somatic or germ cells; those in germ cells are heritable Spontaneous and Induced Mutations • Spontaneous: changes in nucleotide sequence that have no known cause; assumed to be accidental o Often occur during enzymatic process of DNA replication o Rate of spontaneous mutation is low for all organisms o Rate varied between different organisms o Even within same species, spontaneous mutation rate varies between genes • Induced Mutations: result of natural or artificial agents o 1927: Hermann Muller showed that x-rays caused mutations in Drosophila. o 1928: Lewis Stadler reported same thing for barley • Mutation rate: likelihood that a gene will undergo a mutation in a single generation of in forming a single gamete Classification based on location of Mutation • Somatic Mutations: not occurring in germ cells o Not transmitted to future generations o Not detected in phenotype if recessive • Germ-line Mutations: occurring in gametes • Autosomal Mutations: within genes on autosomes • X-linked Mutations: on x chromosomes o Somatic mutation have greater effect in males if they’re x-linked • Dominant mutations that occur in adults are often masked by other nonmutant cells in the same tissue that perform the same function • Mutations in gametes are of greater significance because they are transmitted as part of the germ line and have potential of being expressed in all cells • Inherited autosomal dominant mutations will be expressed phenotypically in first generation • Homogametic female may pass x-linked recessive mutations in hemizygous male Classification Based on Type of Molecular Change • Point mutation or base substitution: change of one base pair to another • Missense: change of one nucleotide of a triplet in a protein-coding portion can result in new triplet and new protein • Nonsense: if triplet is changed to stop codon • Silent mutation: point mutation alters codon but does not change amino acid • Transition: if pyrimidine replaces pyrimidine or purine replaces purine • Transversion: purine replaces pyrimidine or vice versaPCB3063'Exam'4'• Frameshift: loss or addition of single nucleotide; changes how the rest of sequence is read Classification Based on Phenotypic Effects • Loss-of-function: reduces or eliminates function of gene product; can be dominant or recessive; any mutation can cause this effect • Null: complete loss of function • Gain-of-function: gene product with enhanced or new functions; most are dominant • Visible: affect morphological trait; alter normal phenotype • Nutritional: loss of ability to synthesis amino acid or vitamin, in bacteria • Biochemical: sickle-cell anemic and hemophilia; do not always affect morphological characters; effect well being and survival • Behavioral: difficult to analyze; affects behavior patterns • Regulatory: affects regulation of gene expression; inappropriately activate or inactivate expression of a gene • Lethal: interrupts process essential to survival; Tay-Sachs and Huntington • Conditional: expression depends on environment • Temperature sensitive: at a permissive temperature, the mutant gene functions normally but loses function at restrictive temperature • Neutral: can occur in protein coding region or any part of genome and whose effect on genetic fitness of an organism is negligible 14.2 DNA Replication Errors and Slippage • DNA polymerases insert incorrect nucleotides during replication • DNA polymerases can usually correct these errors in exonuclease proofreading • Replication errors due to mispairing lead to point mutations • Tautomers: the different forms that bases can take from a single proton shift • Insertions or deletions occur when one strand of DNA template loops out (polymerase misses looped out nucleotides) or when DNA polymerase slips • Replication Slippage: can occur anywhere; common in regions with repeated sequences Tautomeric Shifts • Keto-enol forms of thymine and guanine and amino-imino forms of cytosine and adenine are biologically important • Tautomeric shifts can lead to permanent base-pair changes and mutations • End result is point mutation Depurination and Deamination • Depurination: loss of one of nitrogenous bases in an intact double-helical molecule; usually a purine • Apurinic State: glycosidic bond linking 1’C of deoxyribose and number 9 position of purine ring is broken; if not repaired DNA polymerase may insert nucleotide a random at that sitePCB3063'Exam'4'• Deamination: amino group in C or A is converted to a keto group; C to U and A to hypoxanthine; alteration in base pairing specifies of these two bases in DA replication Oxidative Damage • DNA can suffer from byproducts of normal cellular processes • Reactive oxygen species generated in normal aerobic respiration • Reactive oxidants: hydroxyl radicals, superoxides, and hydrogen peroxide; can produce over 100 modifications n DNA 14.3 • Mutagens: have potential to damage DNA and cause mutations • Fungal toxins, cosmic rays, UV light, industrial pollutants, X-rays Base Analogs • Base analogs: mutagen that can substitute for purines or pyrimidines during nucleic acid biosynthesis • 5-BU: 5-bromouracil; behaves like thymine; halogenated at 5 position of pyrimidine ring; if linked to deoxyribose BrdU is formed • Presence of bromine atom in place of methyl increases possibility that Tautomeric shift will occur • 5-BU also increases UV light sensitivity • 2-amino purine (2-AP): acts as analog of adenine; base pairs with thymine and cytosine Alkylating, Intercalating and Adduct-Forming Agents • Alkylating agent: donate alkyl groups to amino or keto groups in nucleotides o Alters base paring, leading to mutations o EMS alkylates keto groups in 6 position of guanine and 4 position in thymine • Intercalating agents: chemicals that have dimensions and shapes that allow them to wedge between base pairs of DNA o Cause base pairs to distorts and DNA unwinds • Adduct-Forming Agents: covalently binds to DNA, altering conformation and interfering with replication and repair o Acetaldehyde in cigarette smoke o Heterocyclic
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