Download Modes of Prokaryotic Genetic Exchange

Modes of Prokaryotic Genetic
Exchange
Three modes of genetic exchange:
1. Transduction- Virus introduces new genes to the host
(other than viral genes)
a. Specialized transduction- only introduces new genes
adjacent to integration site of virus (gal and bio)
b. Generalized transduction- can introduce any part of host
genome
2. Conjugation- DNA is moved from one cell to another
based on cell-cell contact or “bridge-like” structures
a. F-plasmid- transfer ability to conjugate
b. Hfr- can transfer entire host genome
c. F’- can transfer some host genes
3. Transformation- taking up DNA from surroundings
Generalized transduction: P1 phage
The phage can take up any DNA that is about the same size as
it’s genome.
Bacterial Conjugation: Overview
• DNA is moved from one
cell to another based on
cell-cell contact or “bridgelike” structures
• The F-plasmid contains the
genes needed for bacterial
conjugation (pilus
formation, cell attachment,
DNA movement)
Hfr strains: Contain integrated Fplasmid
= origin of transfer
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Hfr strain
F’ plasmid is inserted into host genome via homologous
recombination (usually at transposon)
Integrated plasmids produce proteins needed for conjugation.
Integrated plasmid is still competent for transfer, and therefore can
transfer entire host genome!
In order to be expressed, the genes transferred from the Hfr strain
must undergo homologous recombination into the recipient
genome
Hfr Strains: Interrupted Mating
Experiments
• Transfer of DNA by
conjugation is constant.
• Allow mating, and then
interrupt it by blending
cells, and see which
genes are transferred.
• The distance between
the genes is in minutes,
indicating how many
minutes it took the
gene to transfer.
F’ Plasmid: Transfer of host DNA by
imprecise excision
• If the F-plasmid is
excised from the host
genome, the excision
can be imprecise,
leading to transfer of
host genes.
Conjugation of F’
plasmid to new cell
Types of Bacterial Conjugation
Mediated Transfer
STRAINS
F+
Hfr
F’
What is it?
Contains F-plasmid
(genes that allow
for conjugation
only).
F-factor integrated
into host genome
via recombination.
F-plasmid plus host
genes on F-plasmid
from imprecise
excision
Frequency of
recombinants
during mating
experiment?
Extremely high
~10-3
High
~10-6
Extremely high
~10-3
What is
transferred?
F-Factor only
Donor chromosome Donor genes
How to get stable
transfer of donor
genes.
No recombination
Recombination into
host chromosome
No recombination
(genes on plasmid)