2. Transduction
• Definition: Gene transfer from a donor to a
recipient by way of a bacteriophage
• Bacteriophage (phage): A virus that infects
bcteria
3. Phages as DNA carriers
Bacteriophages are natural vectors that
transduce DNA from one bacterial cell to
another.
A bacteriophage cannot “live” or reproduce
without getting inside a bacterial cell
5. Phage Composition and Structure
• Composition
– Nucleic acid
Head/Capsid
– Protein
Contractile Core
Sheath
• Structure (T4) Tail Fibers
– Size (80 X 100 nm)
Base Plate
– Head or capsid
– Tail
6. Infection of Host Cells by Phages
• Adsorption
–Tail fibers
– Receptor is LPS for T4
• Irreversible attachment
– Base plate
• Sheath Contraction
• Nucleic acid injection
• DNA uptake
7.
8.
9.
10. Types of Bacteriophage
• Lytic or virulent – Phage that multiply within the host
cell, lyse the cell and release progeny phage (e.g. T4)
• Lysogenic or temperate phage: Phage that can either
multiply via the lytic cycle or enter a quiescent state
in the bacterial cell.
– Expression of most phage genes repressed
– Prophage – Phage DNA in the quiescent state
– Lysogen – Bacteria harboring a prophage
12. Transduction
• Types of transduction
1. Generalized - Transduction in
which potentially any donor
bacterial gene can be transferred.
2. Specialized- Transduction in which
only certain donor genes can be
transferred
13. Generalized Transduction
• Infection of Donor
• Phage replication and degradation of host DNA
• Assembly of phages particles
• Release of phage
• Infection of recipient
• Homologous recombination
Potentially any donor gene can be transferred
16. Specialized Transduction
Lysogenic Phage
• Excision of the prophage
• Replication and release of phage
• Infection of the recipient
• Lysogenization of the recipient
– Homologous recombination
also possible
17. Specialized Transduction
• In specialized or restricted transduction, the
transducing particle carries only specific
portions of the bacterial genome.
• Specialized transduction is made possible by
an error in the lysogenic life cycle.
18. • When a prophage is induced to leave the host
chromosome, excision is sometimes carried out
improperly.
• The resulting phage genome contains portions of
the bacterial chromosome (about 5 to 10% of the
bacterial DNA) next to the integration site.
• A transducing phage genome usually is defective
and lacks some part of its attachment site. The
transducing particle will inject bacterial genes
into another bacterium, even though the
defective phage cannot reproduce.
19. • The best-studied example of specialized
transduction is the lambda phage. The lambda
genome inserts into the host chromosome at
specific locations known as attachment or att
sites.
• The phage att sites and bacterial att sites are
similar and can complex with each other.
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23. • Normal out looping
• Phage excised out
• Rare abnormal outlooping
• dgal
• Defective in gal,
• defective in integration site
• Helper phage – hybrid attachment site
