Warm-Up

• What is bacterial transformation?

Plate 28Bacterial 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(+)

http://upload.wikimedia.org/wikipedia/commons/3/3e/Conjugation.svg

• 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

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/animations/conjugation/conj_frames.htm

Transformation vs. Conjugation