S57

1
S57
Ch14 Gene Transfer in Bacteria

• Why study gene transfer in bacteria?
• It happens
• It leads to increased drug resistance in bacteria
  of medical relevance (note gonorrhea story in
  text!)
• It is an experimental key for advances in the
  50-70s, the golden years of molecular biology
• (Valuable genetics teaching tool- simple enough
  to teach and to develop problems)

Evolutionary relationships Fig 14.1
2
S58
A few bacteria factoids
Fig 14.3
Live anywhere and everywhere (thermal vents, hot
springs, anaerbic, gut) Some eat oil( ) Size
1um (yeast 10um, human cells 100um) Very easy and
cheap to grow
Fig 14.2
3
S59
Genome
Fig 14.6

• Circular-no ends!
• 4x106 basepairs (4000kb, 4Mb)
• Bacteria usually haploid (1 copy per cell)
• 4000 genes (1/10th of human cells)
• Dense 90 coding, 10 junk
• (Bacteriophages, transposons, selfish DNA)

4
S60
Plasmids
Fig 14.8

• Smaller than genome (4000-100,000bp)
• Replicated by proteins from host cell
• Drug resistance genes
• Probably selfish DNA

5
S61
Three mechanisms of gene transfer in bacteria
We will discuss further
We will discuss further
Not discussed further
6
S62
Conjugation and F factor

• F factor (fertility factor)- a sophisticated
  plasmid
• OriT, Origin of transfer
• insertion sequences,
• genes for own replication,
• genes for own transfer
• History

7
S63
Conjugation cycle
Conjugation converts an F- to F, and original F
remains F Note no host genes (in blue)
transferred from F to F-
8
S64
To transfer host genes, need an Hfr
strain (high frequency)
Homologous Recombination here
One cell will have one or another of these F to
Hfr relative low frequency (few ),
9
S65
Using an Hfr for gene transfer
10
The Waring-Blender experiment
S66
Cross Hfr thrstrSaziRtonRgallac x
F-thr-strRaziStonSgal-lac- donor
recipient
StrR in plate
11
S67
Results from different Hfrs
Genome genetic distances noted in minutes
12
S68
3 point cross can be slightly tricky
Only even number of crossovers generate an intact
circular recipient chromosome, and thus a viable
cell!! Relative numbers indicate relative
frequency, and each recombination (crossover) is
independent and of low frequency.
13
S69
Fan oft-used genetic trick
14
S70
Table 14.2 Summary of F
15
S71
A second form of gene transfer- transduction
Fig 14.19
Fig 14.20