Class Note for MIC 205A at UA 3
Class Note for MIC 205A at UA 3
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This 4 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Arizona taught by a professor in Fall. Since its upload, it has received 16 views.
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Date Created: 02/06/15
Microbial Genetics Mutations Genetic Variation I Many differences among microbes are the result ofmutation or recombination followed by selection I Mutation A change in the base sequence ofthe DNA I Mutations may be bene cial neutral or harmful I Mutations are changes in the genotype which may or may not affect the phenotype Types of mutations I Base substitutions I The most common type ofmutation I A single base pair is replaced by another I Frame shilt mutations I One or more base pairs are inserted or deleted in the DNA I Results in a change in the reading of codons Consequences of base substitutions I Silent mutation base change results in no change ofthe amino acid sequence ofthe translated protein I Silent mutations have no effect on phenotype I A result ofthe fact that multiple codons can code forthe same amino acid I E g AGU and ABC buth nude terSehhe Consequences of base substitutions I Missense mutation base change results in the change of an amino acid in the translated protein innmil lTl 1 quotquotW 6961 E Consequences of missense mutations I The amino acid substitution induced by the missense mutation may have no effect on the Jnction ofthe protein OR I It may abolish the activity ofthe protein or alterits function having an effect on phenotype Example Sickle eeu msease in humans is due tn a rmssehse mutater if the gehetur gl bln As a result the shape uf red bland cells is altered atteethg their muvementthruugh apma es Consequences of base substitutions I Nonsense mutation base change generates a stop codon in place ofthat coding for an amino acid I Results in production ofa truncated protein Usually results in a nonfunctional protein mm W hostilityth Consequences of frameshift mutations Frameshlft mutation addition ordeletion orone or more oases Results in misreading ortne codons cnanged reading rrame Almost always results in long stretcnes oraltered amino acids an tne production Ofll lactlve protein il 1 1 IT mm 999 n E Causes of mutations l Spontaneous mutations I Occur in the absence ofmutation causing agents I Due to occasional mistakes in DNA replication I Induced mutations I Caused by mutagens agents such as chemicals and radiation which induce mutations Chemical mutagens Example Nitrous acid alters adenine such that it pairs with cytosine instead ofthymine Chemical mutagens I Example Ethidium bromide inserts between bases causing frameshi mutation I Example Nucleotide analoguessubstitute for a base but have different pairing properties Some can be used as antiviral ortumourtherapy Radiation I Ionizing radiation eg Xrays and gamma rays Causes the formation of ions that can react with nucleotides causing base changes and the deoxyribosephosphate backbone causes chromosomes to break viiiiamiigiii UV radiation lnduees rormation or cuvalerlt bonds between adiaeenttnymines tn farm thymlhe dimers wnien ean hut be replicated DNA repair I Prokaryotic and Eukaryotic cells have the capacity to 39 repair DNA Weti DNA pnutulyase reccignizes and repairs thymine girners using energyfrum iignt iignt repalr pncitcireactivaticin A general EXElSlEIrl repair A system excises inccirrect bases leaving a gap rcir DNA W pulymerase l and iigasetciriii 9 Frequency of mutation l Mutation rate is the probability that a gene will mutate when the cell divides l A spontaneous mutation rate for Ecoi 1 in 109 replicated base pairs or1 in 10E replicated genes I Mutagens increase the mutation rate by 101000 times to as high as 1 in 103 replicated genes Medical importance of mutations I Bacteria can mutate to more virulent forms more potent toxins slimier capsules I Bacteria can mutate to antimicrobial resistance or already resistant bacteria can achieve greater resistance Important in hospitals and animal feedlots where there is constant exposure to antibiotics I Once present genes forvirulence characteristics and antibiotic resistance can be transferred to other bacteria Identifying bacterial mutants I Given a mutation rate OH in 106 genes in large bacterial populations a few mutants will be present I Mutants can be detected by Pcisitive direct seiecticin Wnere rnutant cells grcivv cir appear different e g i peniciiiin resistant mutants can be iuentnieu by expusuve tci peniciiiin The mutant cellswill gmwthe Wiiutype cellswill nut Negative indirect selecticin detects rnutant cells because they uci ncit grcivv e g i mutants in nistiuine blusynthesis canncit grew in the absence cit nistldlnE butthe Wiiutype cells can Negative selectionreplica plating M x 53 i133 neiiimi Mum tii 91312 t m l miragenitiitinmuiimmii x I x gunmen The Ames Test for Chemical Carcinogens I Basis ofthe Ames test Uses Salmonella nistiuine picisyntnetic mutants nistiuine requiring Mutagens WlH cause rEVErSan cir back rnutaticin cit these mutants tci allthhEmtu grcivv in tne apsence cit nistiuine l About 90 ofthe substances identi ed as mutagenic bythe Ames test have been con rmed as carcinogenic in animals I In general the more mutagenic the substance the more carcinogenic it has been found to be The Ames Test for Chemical Carcinogens I In the r a presence ofa H carcinogen the his39 Salmonella will back mutate to his Microbe of the Week Salmonella enneritidl39s I Member ofthe Enterobacteliaceae like E coll I Gramnegative bacillus I Can be found in the GI track of chickens other food animals I Causes gastroenteritis in humans and animals I Invades intestinal cells I A food borne pathogen Salmonella typhi a highly virulent Salmonella species that causes typhoi fever in humans only
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