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Bacterial Recombination

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Model Or anism E. Coli

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Bacterial colonies, each derived from a single cell

Figure 5-3

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Mixing bacterial means mixing genotypes and rare recombinants are observed.

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Mixing bacterialgenotypesproduces rarerecombinants

Figure 5-5b

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If recombination occurs, then it cannot be due tocongugation

Fine enough to preventcells but not DNA orviruses

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Bacteria exchange DNA by several processes

Figure 5-2

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Bacteria conjugate through pili a cell surface appendage, itssynthesis controlled by genes on a small circular moleculecalled the F factor.

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Transfer is not reciprocal: a donar F+ (Fertility) and arecipient (F-)

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F plasmids transfer during conjugation using therolling circle mechanism the circular F factor

rolls, unwinds one strand of DNA.

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Two types of DNA transfer can take

place during conjugation

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F factor integrates into thedonar genome changing thecell into a Hfr cell.

F factor unrolls, dragging

the donar genome with itand moves into the host.

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Partial diploid or merozygote

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Episomes exist in 2 states: (1) autonomous (2) integrated

2 kinds oftransfers,2 states

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Transfer initiated at OriT, within the Ffactor The origin and then chromosomalDNA is transferred first, the rest of Ffactor last.The F factor can orient in

different directions depending on pairing

A single crossover inserts F at a specific locus, whichthen determines the order of gene transfer

Hfr cells rarely convert thehost to Hfr

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The order remainsthe same but, thedirection of transfermay differ,depending on thesite of integration,more accurately theorientation of the Ffactor

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Tracking time of marker entry generates a chromosomemap:

(A) Cross strain 1 and 2 (mix) (1)HFrthr+ leu+ azir tonrlac+gal+strs

(2)

F- thr- leu- azis tonslac- gal- strr

+ = wild, s = sensitive,- = defective r= resistant

(B) Plate onto media containing:(1)streptomycin -kills strscells,

(2) lacking threonine and leucine

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Broad Scale Mapping Tracking time of marker entrygenerates a chromosome map

Late -few

Early - many

What is the order ofgenes ?

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!! #$ %&'()*+ ,-./012( '. #3 ,0&& 2- '. #$

4! 5 %'16'& 781 8-10*9. ,:1-)-(-)0 ; ! #$!%&'() ,1-((0(

81-) '. ' 781 ,0&& 2- ' 10,*%*0.2 ,0&& %'12 -8 2:0 81'9)0.2 10,-)?*.0(; 2:0

,1-(( *( .-2 10,*%1-,'& '.+ 2:0 10,*%*0.2 2 *( 10,-)?*.'.2;

@:010'( 2:0 10,-)?*.'.2 81'9)0.2 *( +091'+0+

#$% &'()* +,- .)/0123 4-(00*0 !"#$%#5

#/%

6(7 287*9-/7*3 (- 287*9-/7* /83 4)*/8): *;420* < .(002=): /

%'16'& +*%&-*+ > )01-AB9-2087*9-/7* /83 *;420* ?27' / ,-/91*87 (, 7'* 8*29'=(-289

-*42.2*87 @6A < 1/: 4/--: ./-7 (, 7'* =/47*-2/) .)/0123

7'*8 27 20 4/))*3 /8

&' )*$+,!- /83 1*-(B:9(7*5

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Fine Scale Mapping Parts of the transferred donor fragment maybe integrated into the host genome through a double crossover

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Linear fragments are degraded

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F Plasmid - a plasmidcarrying bacterial DNA.

Produced by outlooping

Cause stable partial

diploids (merozygotes)in lineages of E coli

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Transformationbacteria may take fragments of DNA fromtheir environment and integrate part of them into theirgenome, transforming their native genotype if there isrecombination.

CD

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Transformation: Mechanism of DNA uptake (linearfragment) by bacteria

25REQUIRES A DOUBLE

CROSSOVER

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Remember, the closer 2 genes, the less likely they will

recombine

If pieces of chromosome do not recombine into the hostchromosome they will be degraded. Most linear

segments of DNA are degraded.

A single crossover opens a circular chromosome, 2 (oran even number) keeps it closed. .

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OTHER EPISOMES

TRANSFERRED DURINGCONGUGATION

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Multiple resistance R- Plasmids. A plasmid with segments from many former bacterial

hosts

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Generalized Transduction

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Structure and function of phage T4

Figure 5-22

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Electron micrograph of phage infection

Figure 5-24

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Cycle of a phage that lyses the host cells

Figure 5-25

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A phage cross made by doubly infecting the host cell withparental phages

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Plaques from recombinant and parental phage progeny

Figure 5-28

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Generalized transductionby random incorporation of bacterial

DNA into phage heads, during phage assembly

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Phages pick up randompieces of donor DNA

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From high cotransduction frequencies, close linkage is inferred,

Alternatively, low cotransduction, distant linkage is inferred

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Specialized transduction moves(transduces) only small segments

on either side of where the

prophage integrates into the hostgenome and then drags flanking

areas with it when it excises

(leaves) in a process similar to Fplasmids that incorporate host

DNA

Phages integrate into the host genome

$D