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biol139-lecture35-2011 - Next topic Next Genes and mutation...

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Unformatted text preview: Next topic Next Genes and mutation pp 207 - 226, 241-243 pp 377 - 397 igenetics Adaptation Versus Spontaneous Mutation Spontaneous Spontaneous Mutation Theory: • supporters argue that mutations occur randomly randomly • at any time in population of cells some cells have undergone 1 mutation whicheven though rNEVER bto T exposed to T1 esistant • resistant make them NEVER een phage • when T1 added, T1-resistant bacteria selected for selected A. Subculture + excess T1 phage Pick resistant colony re-infect - still resistant plate spontaneous E. coli culture (wild-type) Phage-resistant E. coli Mutations are chance occurrences modifying the genome (spontaneous mutations) 1943 Salvador Luria and Max Delbruck used E. coli and T1 phage to test hypothesis regarding mechanisms responsible for appearance of a new phenotype (adaptive vs spontaneous) Luria-Delbruck ‘Jackpot’; fluctuation test Set up small vials of bacterial cultures and let them grow for a specified amount of time –then added equal portions of each culture to plates containing media and phage. Media and phage plates adaptation Hypothesis 1 Resistance is the result of active adaptation to the selective pressure (phage). Every bacteria has a small but constant chance of acquiring resistance upon exposure to T1 phage. If true, the number of resistant cells should only depend on the # of bacteria and phage. Therefore if constant amount of bacteria and phage are added Therefore and incubation time is constant, then all plated cultures should adapt similarly, resulting in similar # of resistant colonies on the plates (no fluctuation) Single cell Single cell Hypothesis 2 Hypothesis Resistance is a result of spontaneous, random mutations exclusive of the selective pressure (phage) exclusive If so, then there will be differences in the number of resistant colonies when plated if due to random mutation This is because the mutation to T1 resistance can occur randomly anywhere during the culturing process and does not require exposure to T1 i.e. if mutation occurs late before addition of phage (generation 3), then will have less resistant cells prior to plating (2 out of 16); however, if mutation occurs early (generation 1), then will have more resistant cells prior to plating (8 out of 16) jackpot jackpot Conclusions: -resistance occurs as a result of random, spontaneous mutations that happen at various time points BEFORE exposure to selective agent. Mutations occurring early in growth of the liquid culture, resulted in many resistant colonies when plated; those occurring late resulted in few resistant colonies (fluctuations in # obtained) Spontaneous mutations occur naturally Mutations Mutations -mistakes in DNA occur at a low frequency all the time -natural processes of deanimation, UV light, mistakes by -natural DNA polymerase DNA Mutagens induce mutations -any physical or chemical agent that raises the frequency of -any mutations above the spontaneous rate above e.g., X rays, base analogs, intercalators e.g., ethidium bromide Examples of How Spontaneous Mutations Occur Spontaneous Processes which can change the information stored in DNA Processes Depurination: • removal of A or G from DNA • during replication can’t specify base • wrong base added about 3/4 time Deamination Deamination: Deamination: • removal of amino group • cytosine to uracil • called Transition Mutation Transition Examples of Induced Mutations Induced e.g. UV, X rays, base analogs, intercalators Radiation Induced Mutations Formation of Thymine dimers by UV Formation of Thymine Dimers Mistakes during DNA replication can alter genetic information but also be repaired by excision of either bases or nucleotides but Nucleotide excision repair removes thymine dimers xeroderma pigmentosum -autosomal recessive -autosomal -mutation resulting in inactive endonuclease -excision repair cannot occur -no treatment available Repair of DNA Damage “mutations = DNA damage – DNA repair” Mistakes during DNA replication can alter genetic information but also be repaired directly! but Both I and III Methyl-directed mismatch repair Methyl-directed Bacterial recognition and repair system that can fix Bacterial mutations after DNA replication -relies on ‘tagging’ after parental strands with methyl groups methyl -eukaryotic cells also have a mismatch repair system, -eukaryotic but the ‘tag’ is not yet known but Parental consensus sequence is 5’ GATC 3’ with A tagged with Methyl Mutations that cannot be corrected…. Unequal crossing-over and transposon movement can change the information content of DNA Mutations that cannot be corrected…. One chromosome ends up with a duplication while the other ends up with a deletion Unequal crossing-over and transposon movement can Unequal change the information content of DNA Unequal crossing over or transposon movement are not Unequal susceptible to excision or mismatch repair susceptible Exam: Bring your Watcard or you will not be allowed to write Exam: Format: around 90-95 questions (end of lecture 35) Majority multiple choice, one very short answer or problem Roughly 25-30% of the questions will be from the first part of course till the 2nd midterm. Bulk on later part of course Includes readings in Text and lecture material Know the names of more important researchers such as Morgan Luria etc and their contributions. Information to be excluded from exam: excluded Any general topics not covered directly in lecture will NOT be included on the final exam (i.e. transposable elements, population genetics etc). You will NOT be responsible for learning these topics on your own. Office Hours Before Exam: I will be in my office during the exam period, but would be best to Email first to let me know your coming ...
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