Medical Microbiology

TheGeneticVariabilityofBacteria 171

chemicallypurefromadonorintoareceptorcell.Intransduction,bacte-

riophagesserveasthevehiclesforDNAtransport.Conjugationisthetransfer

ofDNAbymeansofcell-to-cellcontact.Thisprocess,madepossiblebycon-

jugativeplasmidsandtransposons,canbeahigh-frequencyoneandmay

evenoccurbetweenpartnersofdifferentspecies,genera,orfamilies.The

transferprimarilyinvolvestheconjugativeelementsthemselves.Conjugative

structurescarryingresistanceorvirulencegenesareofconsiderablemedical

significance.

Theprocessesofrestrictionandmodificationareimportantfactors

limitinggeneticexchangeamongdifferenttaxa.Restrictionisbasedon

theeffectsofrestrictionendonucleasescapableofspecificexcisionofforeign

DNAsequences.Theseenzymeshavebecomeinvaluabletoolsinthefieldof

geneticengineering. &

MolecularMechanismsofGeneticVariability............

SpontaneousMutation................................

Intheyear 194 3,LuriaandDelbru ̈ckusedtheso-calledfluctuationtestto

demonstratethatchangesinthecharacteristicsofbacterialpopulations

weretheresultsofrare,randommutationsinthegenesofindividualcells,

whichthenwereselected.Suchmutationsmayinvolvesubstitutionofasin-

glenucleotide,frame-shifts,deletions,inversions,orinsertions.Thefre-

quencyofmutationsisexpressedasthemutationrate,whichisdefined

astheprobabilityofmutationpergenepercelldivision.Theratevariesde-

pendingonthegeneinvolvedandisapproximately 10 –6to 10 –1^0 .Mutation

ratesmayincreasedrasticallyduetomutagenicfactorssuchasradioactivity,

UVradiation,alkylatingchemicals,etc.

Recombination.........................................

Thetermrecombinationdesignatesprocessesthatleadtotherestructuringof

DNA,formationofnewgenesorgeneticcombinations.

Homologous(generalized)recombination.ApreciseexchangeofDNAbe-

tweencorrespondingsequences.Severalenzymescontributetothecomplex

breakageandreunionprocessinvolved,themostimportantbeingtheRecA

enzymeandanothertheRecBCnuclease.Fig. 1. 2 (p. 1 4)showsanexampleof

homologousrecombinationresultingintheexchangeofminicassettesbe-

tweentwogenes.

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Kayser, Medical Microbiology © 2005 Thieme