Bacterial Gene Transfer

Bacterial Gene Exchange

• General scheme of bacterial exchange of DNA. DNA from a donor cell is transferred to a recipient cell where it undergoes recombinational exchange, replacing one or more of the recipient's genes with those from the donor.

What are Plasmids?

• Plasmids play a role in many forms of DNA exchange between bacteria

• They function like MINI-CHROMOSOMES. • Plasmids are composed CIRCULAR DNA that is

much SMALLER than the genomic DNA • While naturally occuring they are also exploited

by scientists to move genes into bacterial and other cells

What are Plasmids?

• Plasmids vary in size, but most are between 1,000 to 25,000 base pairs vs. 4,000,000 bp in the genome.

• Plasmids REPLICATE AUTONOMOUSLY from the genomic chromosome.

• Often there are MANY PLASMID COPIES present in one cell. Further, a cell may contain SEVERAL DIFFERENT PLASMIDS or it may contain NO PLASMIDS at all. Plasmids generally carry genes that are NOT essential for survival except under certain circumstances

What are Plasmids?

• One example would be genes for ANTIBIOTIC RESISTANCE

• Some plasmids carry resistance genes to several antibiotics, making them very dangerous pathogens.

• VIRULENCE-PLASMIDS enhance a bacteria’s ability to cause disease

• These two together are a “bad” combination

Three Forms of Bacterial gene exchange

• TRANSFORMATION- “naked DNA”

• CONJUGATION- involves transfer via pilli

• TRANSDUCTION- involves transfer from a Phage (virus)

TRANSFORMATION

• We leaned about Tranformation with Frederick Griffith and Oswald Avery.

• Non-virulent Rough Pneumonia bacteria transformed to virulent by DNA of Smooth virulent bacteria

How Transformation Works

ConjugationThe discovery of CONJUGATION, the

ability of bacterial cells to transfer DNA between cells that are in physical contact

Basic conditions for conjugationDonor cells carry a unique plasmids called FERTILITY

or SEX PLASMIDS. Cells with fertility plasmids are called F+ or MALE cells, whereas cells, lacking the sex plasmids are said to be FEMALE or F-.

• The sex plasmid genes are responsible for the synthesis of special pili called SEX PILI. Sex pili are thin long, hollow protein tubes that have "sticky" RECEPTORS on their ends that bind firmly to molecules (ligands) on recipient cell walls.

Basic Conditions for Conjugation

• Following the ATTACHMENT of the two cells by the pili, they form a "CONJUGATION BRIDGE” through which DNA can pass.

• An enzyme CUTS one strand of the donor's DNA at a unique site and a newly synthesized stand of DNA passes through the conjugation bridge INTO THE RECIPIENT CELL.

• This newly synthesized strand of donor DNA is converted to a DOUBLE STRANDED form which is able to EXCHANGE into the host's DNA by recombination.

Fertility Plasmid

Representative FERTILITY PLASMID. A fertility plasmid carries the genes for conjugation as well as a number of other genes. In this figure the fertility plasmid also carries antibiotic resistant genes.

TRANSDUCTION

The third way of transporting DNA between organisms involves the

mediation of viruses. Bacterial viruses are called BACTERIOPHAGE or just

plain PHAGE by most microbiologists.

Animations of DNA Transfer

http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter13/animation_quiz_1.html