chapter 13 -- mechanisms of genetic variation

chapter 13 -- mechanisms of genetic variation - Chapter 13:...

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Chapter 13: Mechanisms of Genetic Variation
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Mutations Errors in genetic code Because bacteria are haploid, mutations often felt immediately rather than being masked by non- mutated member of chromosome pair Genotype is the genetic constitution of a cell: what genes are present and the characteristics they confer Genotype is changed by mutations Phenotype is observed expression of genotype; which traits are actually expressed Phenotype may or may not be changed by a mutation
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Types of Mutation Spontaneous mutations: Can arise during DNA replication DNA polymerase III has a proofreading function to reduce the frequency of mutations Tautomerization of bases, transposition Induced mutations: Can result from exposure to certain environmental agents (mutagens) UV light, mustard gas, ionizing radiation Base analogs, DNA modifying agents, intercalating agents
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Mutagens React chemically with DNA to modify or remove groups involved in H bonding and results in pairings of A to G, etc. Four types of chemical mutagens: UV light causes adjacent thymine residues to dimerize Base analogs and tautomers- similar to normal bases but base-pair poorly and copy improperly Intercalating agents are flat molecules that slip between base pairs Transposons are mobile DNA elements
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Tautomerization & Base Analogs Tautomers = structurally similar to base but has an added side chain such as bromine that interferes with normal base pairing Nucleotides can modified by chemicals which are attracted to negatively charged molecules Often these are alkylating groups Modification of nucleotide results in improper base pairing during replication De-amination of nitrogenous base also affects H bonding
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Tautomers and Base Analogs Bromine base  Bromine base  analogs of uracil analogs of uracil Tautomers =  Tautomers =  Imino or enol form  Imino or enol form  instead of keto  instead of keto  form form
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Thymine Dimers Block DNA replication due to covalent bond between T residues Complementary nucleotides cannot base pair by H bonds to synthesize new strands Replication sometimes proceeds, introducing a mutation in to the strand complementary to the dimer Also interferes with transcription of mRNA if the dimer is in the template strand
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Intercalating Agents About the same size and shape as a base pair Disrupt proper replication No H bond to break to form replication fork Can cause frameshift mutations to occur by some unknown mechanism DNA staining dyes like ethidium bromide are an example Intercalating agent with  Intercalating agent with  (covalent bonds between rings) (covalent bonds between rings)     Normal H bonding of bases Normal H bonding of bases
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Types of Mutations 1. Point mutations affect one codon only Missense mutations alter codon to specify different amino acid ACU (thr) CCU (pro)
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chapter 13 -- mechanisms of genetic variation - Chapter 13:...

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