(b)Mutationsoccurpriortopenicillinexposure107coloniesofMakethreereplicaplates.penicillin-sensitive bacteriaIncubatetoallowpenicillinresistantcoloniesto grow.MasterplateNopenicillininmediumPenicillininmediumVelvetPenicillininmediumPenicillininmediumPenicillin-resistantcoloniesgrowinthesamepositiononallthreeplates.Fig.7.5b7-16
Fig. 7.5b Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 7-16
Interpretation ofLuria-Delbrick fluctuation test andreplicaplatingMutations do not arisein particulargenes as a directresponse to environmental change.Mutations occur randomly at any time.Bacterial resistance arises from mutations that existbefore exposureto bacteriocide.After exposure to bacteriocide,the bacteriocidebecomes a selective agentkilling off nonresistant cellsallowing only the preexisting resistant cells to survive.7-17
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 7-17 Interpretation of Interpretation of Luria-Delbrück fluctuation test and fluctuation test and replica plating replica plating Mutations do not arise in particular genes as a direct Mutations do not arise in particular genes as a direct response to environmental change. response to environmental change. Mutations occur randomly at any time. Mutations occur randomly at any time. Bacterial resistance arises from mutations that exist Bacterial resistance arises from mutations that exist before exposure to before exposure to bacteriocide bacteriocide. After exposure to After exposure to bacteriocide bacteriocide, the bacteriocide bacteriocide becomes a selective agent killing off nonresistant cells, becomes a selective agent killing off nonresistant cells, allowing only the preexisting resistant cells to survive. allowing only the preexisting resistant cells to survive
Where do mutations come from?7-18
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 7-18 Where do mutations come from?
Deamination removes-NH2Hydrolysis of a purinebase,AorGoccursgroup.Can change CtoU.inducing a substitution of C-G to1000 times an hour inevery human cell.T-A after replication(b)Deamination(a)DepurinationNormalsequenceAminogroupTDeaminationApurinicsiteReplicationOHGuanineCytosineUracilGuanineReplicationreleasedMutantsequence7-19Fig.7.6 a,b
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 7-19 Hydrolysis of a Hydrolysis of a purine base, A or G occurs base, A or G occurs 1000 times an hour in 1000 times an hour in every human cell. every human cell. Deamination Deamination removes removes –NH2 group. Can change C to U, group. Can change C to U, inducing a substitution of C inducing a substitution of C-G to T-A after replication. A after replication. Fig. 7.6 a,b
Oxidation from free radicals formed by irradiation damagesindividual bases.(e)OxidationNormalsequenceActiveoxygenspecies+Oxidativedamage?HGuanineReplication8-oxodG(GO)GCMispairingwithAReplicationMutant sequence7-20Fig.7.6e
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 7-20 Fig. 7.6 e Oxidation from free radicals formed by irradiation damages individual bases