07 plasmids mutations.ppt

07 plasmids mutations.ppt - CEE 266 ENVIRONMENTAL...

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Unformatted text preview: CEE 266 ENVIRONMENTAL BIOTECHNOLOGY Lecture 7 (Chromosomes, Plasmids, Mutations) Circular Linkage Map of the Chromosome of E. coli K-12 Figure 11.1 Genetic Map of the E.Scherichia coli Chromosome  Some Features of the E. coli Chromosome   Many genes encoding enzymes of a single biochemical pathway are clustered into operons   Operons equally distributed on both strands   ~ 5 Mbp in size   ~ 40% of predicted proteins are of unknown function   Average protein contains ~ 300 amino acids Plasmids: General Principles  Plasmids: genetic elements that replicate independently of the host chromosome   Small circular or linear DNA molecules   Range in size from 1 kbp to > 1 Mbp; typically < 5% of the size of the chromosome   Abundance (copy number) is variable Biological Significance of Plasmids   Removal (curing) plasmids from host cells can result from various treatments   Plasmids have been widely exploited in genetic engineering   Genetic information encoded on plasmids is not essential for cell function under all conditions but may confer a selective growth advantage under certain conditions The Bacterial Chromosome and Bacterial Plasmids Figure 11.2 Examples of Phenotypes Conferred by Plasmids Examples of Phenotypes Conferred by Plasmids Mutations and Mutants   Mutation   Heritable change in DNA sequence that can lead to a change in phenotype (observable properties of an organism)   Mutant   A strain of any cell or virus differing from parental strain in genotype (nucleotide sequence of genome)   Wild-type strain   Typically refers to strain isolated from nature Mutations and Mutants   Selectable mutations   Those that give the mutant a growth advantage under certain environmental conditions   Useful in genetic research   Nonselectable mutations   Those that usually have neither an advantage nor a disadvantage over the parent   Detection of such mutations requires examining a large number of colonies and looking for differences (screening) Molecular Basis of Mutation   Induced mutations   Those made deliberately   Spontaneous mutations   Those that occur without human intervention   Can result from exposure to natural radiation or oxygen radicals   Point mutations   Mutations that change only one base pair   Can lead to single amino acid change in a protein or no change at all Possible Effects of Base-Pair Substitution Figure 11.6 Molecular Basis of Mutation  Deletions and insertions cause more dramatic changes in DNA  Frameshift mutations   Deletions or insertions that result in a shift in the reading frame   Often result in complete loss of gene function Shifts in the Reading Frame of mRNA Figure 11.7 Molecular Basis of Mutation  Genetic engineering allows for the introduction of specific mutations (site-directed mutagenesis)  Point mutations are typically reversible  Reversion   Alteration in DNA that reverses the effects of a prior mutation Molecular Basis of Mutation  Revertant   Strain in which original phenotype that was changed in the mutant is restored   Two types   Same-site revertant: mutation restoration activity is at the same site as original mutation   Second-site revertant: mutation is at a different site in the DNA   suppressor mutation that compensates for the effect of the original mutation Mutation Rates   For most microorganisms, errors in DNA replication occur at a frequency of 10-6to10-7 per kilobase   DNA viruses have error rates 100 – 1,000 X greater   The mutation rate in RNA genomes is 1,000-fold higher than in DNA genomes   Some RNA polymerases have proofreading capabilities   Comparable RNA repair mechanisms do not exist Mutagenesis   Mutagens: chemical, physical, or biological agents that increase mutation rates   Several classes of chemical mutagens exist   Nucleotide base analogs: resemble nucleotides   Chemical mutagens can induce chemical modifications   I.e., alkylating agents like nitrosoguanidine   Acridines: intercalating agents; typically cause frameshift mutations Nucleotide Base Analogs Figure 11.8 Mutagenesis   Several forms of radiation are highly mutagenic   Two main categories of mutagenic electromagnetic radiation   Non-ionizing (i.e., UV radiation)   Purines and pyrimidines strongly absorb UV   Pyrimidine dimers is one effect of UV radiation   Ionizing (i.e., X-rays, cosmic rays, and gamma rays)   Ionize water and produce free radicals   Free radicals damage macromolecules in the cell Mutagenesis   Perfect fidelity in organisms is counterproductive because it prevents evolution   The mutation rate of an organism is subject to change   Mutants can be isolated that are hyperaccurate or have increased mutation rates   Deinococcus radiodurans is 20–200 times more resistant to radiation than E. coli DNA Repair Systems   Direct reversal: mutated base is still recognizable and can be repaired without referring to other strand   Repair of single strand damage: damaged DNA is removed and repaired using opposite strand as template   Repair of double strand damage: a break in the DNA   Requires more error-prone repair mechanisms ...
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This note was uploaded on 02/02/2012 for the course CEE 266 taught by Professor Shailymahendra during the Fall '11 term at UCLA.

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