Bacteria divide by binary fission which is a form of asexual reproduction (does not involve sex).
1) Transformation - uptake of pure DNA, NO cell to cell contact
2) Transduction - transfer of DNA by viruses, NO cell to cell contact
3) Conjugation - DNA transferred from donor to recipient cell via conjugation bridge
Transformation
Historical: Oswald Avery in 1944 proved that DNA ALONE is the genetic information
How do we get the DNA in? Make the cells competent (able to take up DNA) by:
a) chemical treatment - usually cold shock plus Ca2+ (E. coli)
b) electroporator - high voltage shock (yeast)
What happens to the DNA?
a) restriction - DNA is regarded as "foreign" and is degraded by restriction enzymes
Recombination
-incoming DNA must match host DNA
-incoming genes replace original recipient genes
a) if incoming DNA is linear it must recombine to survive
b) if incoming DNA is:
I) circular
II) has its own origin of replication
then it can survive without recombination
Plasmid
-circular molecule of DNA
-can replicate itself
a) found naturally in many bacteria
b) widely used in genetic engineering
Transduction
BACTERIOPHAGE infects donor cell and destroys it.
-most virus particles carry virus DNA
-1 in 10,000 carry bacterial DNA by mistake
Generalized Transduction by P1
- P1 attacks E. coli
- P1 carries 90 kilobases of DNA (80-90 genes)
- Any particular gene will be carried AT RANDOM by 1 in 500,000 viruses
- Get 1,000,000,000 bacteria per ml so transduction happens quite often
- a few genes are chosen preferentially (e.g. lambda bacteriophage)
Conjugation
Donor = F+
Recipient = F-
The cell-to-cell contact of the donor and the recipient form the sex pilus
DNA moves through conjugation bridge from donor to recipient
F = Fertility
Donor ability is due to F-plasmid
oriV - origin for vegetative replication
tra - transfer genes
oriT - origin of transfer
rep - replication genes
oriV
- replication is bi-directional (like bacterial chromosome)
- replicates in synchrony with bacterial chromosome
oriT
- single strand enters recipient which will synthesize the complementary strand
Chromosomal Transfer by F-plasmid
- genes on F-plasmid (tra genes) specify formation of sex pilus and conjugation bridge
- usually only F-plasmid is transferred
- sometimes chromosomal genes are moved
Insertion Sequence "IS"
- integration (crossing over) of plasmid can occur at matching "IS" sequences
E.coli chromosome has:
7 copies of IS 1
13 copies of IS 2
6 copies of IS 3
F plasmid has:
0 copies of IS 1
2 copies of IS 2
1 copy of IS 3
Hfr - strain ("high frequency")
- F-plasmid is integrated into the chromosome so transfers bacterial genes
- usually only front half of F-plasmid enters recipient and only part of chromosome is transferred
R-plasmids ("resistance")
- carries antibiotic resistance genes
- most R-plasmids cannot transfer the chromosome
Host Range
F-plasmid and related R-plasmids can inhabit enteric bacteria:
E. coli
Salmonella
Shigella
Yersinia
P-plasmids can inhabit most gram-negative bacteria, originally P for Pseudomonas, later P for promiscuous
Incompatibility
- Two related plasmids cannot inhabit the same bacterium (rep genes)
- Two unrelated plasmids OK (fig 8.17)
ColE plasmid
small multicopy plasmids
- not transferable
- may be mob+ (mobilization)
Questions and comments about MICR 302 to: bender@micro.siu.edu
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SIUC / College of Science / Microbiology / courses / MICR302
URL: http://www.science.siu.edu/microbiology/micro302/transfer.html
Last updated: 11-Mar-99 / laa
