Medical Microbiology

TheMolecularBasisofBacterialGenetics 167

TheStructureofBacterialDNA...........................

Abacterium’sgeneticinformationisstoredinitschromosomeandplasmids.

EachofthesestructuresismadeofasingleDNAdoublehelixtwistedtothe

right,thenadditionallytwistedtotheleftaboutitshelicalaxis(supercoiled,

seep. 14 8ff.andFig.3. 17 ).PlasmidsconsistingoflinearDNAalsooccur,

althoughthisisrare.ThisDNAtopologysolvesspatialproblemsandenables

suchfunctionsasreplication,transcription,andrecombination.Somegenes

arecomposedofamosaicofminicassettesinterconnectedbyconservedDNA

sequencesbetweenthecassettes(seeFig. 1. 2 ,p. 1 4).

Chromosome.Thechromosomecorrespondstothenucleoid(p. 14 8ff.).

TheE.colichromosomeiscomposedof4.63! 106 basepairs(bp).Itcodes

for 4288 proteins.Thegenesequenceiscolinearwiththeexpressedgenetic

products.Thenoncodinginterposedsequences(introns)normallyseenin

eukaryoticgenesareveryrare.ThechromosomesofE.coliandnumerous

otherpathogenicbacteriahavenowbeencompletelysequenced.

Plasmids.TheplasmidsareautonomousDNAmoleculesofvaryingsize(3!

103 to4.5! 105 bp)localizedinthecytoplasm.Largeplasmidsareusually

presentinonetotwocopiespercell,whereassmallonesmaybepresent

in 1 0,40,or 100 copies.Plasmidsarenotessentialtoacell’ssurvival.

ResistancePlasmidinEscherichiacoli

Fig.3. 17 aCovalentlyclosedcircle

(CCC),alsoknownasa“supercoil”or

“supertwist.”

bOpencircle.Thisopenformisan

artifactproducedbyanickinone

strandoftheDNAdoublehelix.

3

Kayser, Medical Microbiology © 2005 Thieme