Download Bacterial Conjugation

Warm-Up
• What is bacterial transformation?
Plate 28
Bacterial Conjugation
Gene Sharing
• Transformation – genetic information
passed indirectly from one bacterium to
another
– Cell  Extracellular fluid  Cell
• Conjugation – genetic information
passed directly from one bacterium to
another
– Requires cell-to-cell contact
What do you think?
• How can the sharing of genes benefit
bacteria?
Conjugation Vocabulary
• 2 types of bacteria:
• F(+) bacteria are able to donate DNA
• F(-) bacteria are able to receive DNA
Plasmids
• Plasmid: double-strand loop of DNA,
consists of about 20 genes (also called
the “fertility factor” or “F factor”)
Plasmid Transfer
• F(+) bacteria extends
and attaches its sex
pilus to the F(-)
bacteria
Plasmid Transfer
• F(+) bacteria creates a duplicates its
plasmid (called a “daughter F factor”)
• The new plasmid is transferred to the
F(-) bacteria through the sex pilus
• The F(-) bacteria now becomes F(+)
Plasmid Transfer
F(+)
F(-)
Plasmid Transfer
F(+)
F(-)
Plasmid Transfer
F(+)
F(+)
Chromosome Transfer – 1st Step
• The plasmid (“F factor”) becomes
integrated with the F(+) bacteria’s
chromosome
• F(+) bacteria is now known as a
“high frequency of recombination”
cell (“Hfr” for short)
Chromosome Transfer – 1st Step
Hfr
cell
Chromosome Transfer - 2nd Step
• Just like in plasmid transfer, the Hfr cell
approaches a F(-) cell and extends its
sex pilus, creating a conjugation bridge
• A single strand of the combined DNA is
duplicated
Chromosome Transfer - 2nd Step
Chromosome Transfer – 3rd Step
• The F(-) cell removes an equal amount
of its DNA and inserts the new Hfr DNA
strand into its chromosome
• F(-) is now known as a recombinant F(-)
cell
Chromosome Transfer – 3rd Step
Chromosome Transfer
Hfr cells are formed when the F factor integrates into
the bacterial chromosome. This integration occurs at a
random location.
Chromosome Transfer
The Hfr cell is still able to initiate conjugation with an F(-) cell.
Chromosome Transfer
When DNA transfer begins, the Hfr cell tries to transfer
the entire bacterial chromosome to the F- cell. The first
DNA to be transferred is chromosomal DNA, and the last
DNA to be transferred will be the F factor DNA.
Chromosome Transfer
Transfer of the bacterial chromosome is almost never
complete. Pili are fairly fragile structures, and shear forces
tend to break the pilus, disrupting DNA transfer before the
entire chromosome can be transferred. As a result, the F
factor itself is rarely transferred to the recipient cell.
Benefits of Bacterial
Conjugation
• Bacteria can easily transfer beneficial
genes within a population, such as
antibiotic resistance
• More common in Gram-negative
bacteria
• http://www.hhmi.org/biointeractive/anim
ations/conjugation/conj_frames.htm
Transformation vs. Conjugation