– UV light, lower energy, cause excitation not ionization, UV radiation cause thymine dimers which block DNA replication, and influence structure of double helices Chemicals:Base Analogs– have structures like normal bases, & are incorporated into DNA during replication, diff. enough in shape that they increase frequency of mispairing during replication, ex. 5-bromouracil (thymine analog, bromo at 5 position rather than methyl, bidirectional so can revert back to wild-type) and 2-aminopurine (ONLY REPLICATING)Deaminating agents– cause oxidative deamination of the amino groups in adenine, guanine, and cytosine, rxn converts amino groups to keto groups, and changes the hydrogen bonding potentials of the modified bases, guanine shows no effects b/c it converts to Xanthine which pairs w/ cytosine, ex. Nitrous acid (BOTH)Acridine Dyes– cause frameshift mutations, +ve charged acridines intercalate b/w stacked bp’s in DNA increase rigidity & alter conformation in double helix causing slight bends & kinks during replication, additions & deletions of one to a few bp’s, ex. Proflavine, and acridine orange (may cause second site suppressor mutations) (ONLY REPLICATING) Alkylating Agents: chemicals that donate an alkyl group to other molecules, induce all types of mutations, ex. When it transfers methyl or ethyl groups to bases resulting in altered bp potentials, ex’s, mustard gas (di-(2 chloroethyl) sulfide), ethyl methane sulfonate (EMS), ethyl ethane sulfonate (EES) (BOTH)Hydroxylating Agent: induces only G:C to A:T transition, changes G to A, ex. Hydroxylamine (transition)
- mutations are also carcinogens- Ames Test, simple & cheap, uses histidine deficient (his -) salmonella to screen for potential chemical mutagens, if observation of histidine reversion (his +) then the chemical is mutagenic- Replica Plating:Technique that allows the demonstration of antibiotic resistance prior to being exposed to a bacterium Demonstrates that environmental stress does not direct/cause genetic changes, it simply selects rare pre-existing mutations that result in phenotypes better adapted to the new enviro.- DNA Repair Mechanisms:Light Dependent Repair (Photoreactivation): Carried out by light activated enzyme, DNA photolyase 1.When DNA is exposed to UV light thymine dimers produce covalent crosslinks2.Photolyase binds to the thymine dimer3.Photolyase activated by absorption of blue light 4.Cleavages of crosslinks5.Release of enzymeExcision Repair: Involves three steps, cuts out damaged DNA and replaces it1.Endonuclease or endonuclease containing enzyme complex recognize, binds to, and excises damaged DNA 2.DNA polymerase fills in gap using undamaged complementary strand as template 3.DNA ligase seal the break left by polymerase2 Main Types:1.Base Excision Repair: removes abnormal or chemically modified bases from DNA, DNA glycosylases recognize abnormal bases, each recognizes a specific type of altered base, glycosylases cleave glycolytic bonds and create AP sites, AP
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- Fall '13