Post on 20-Aug-2015
TransductionINSTRUCTOR : DR. KENTON HERNANDEZ
CENTRAL AMERICA HEALTH SCIENCE UNIVERSITYBELIZE
PRESENTEDE BY : Gia K. Sharma
Contents
Historical
Transduction
Mechanism Bacteriophages and its life cycles
Type of transduction Generalized Transduction
Specialized Transduction
Significance
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historicalLederberg & ZinderTransduction was first discovered in 1952 by Joshua Lederberg and Norton Zinder
Joshua Lederberg Norton zinder
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How transduction was discovered
Studied in Salmonella typhimurium
Plated two auxotrophic strains (LA-2 and LA-22) individually on minimal medium, no cells grew.
Plated a mixture of the two auxotrophic strains on minimal medium, cells grew into colonies.
Thus, genetic exchange was taking place between the two cell types.
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U-tube Experiment
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Unexpected discovery
U-tube Experiment
Performed U-tube experiment .
Found that part of the cells on one side of the U-tube were
prototrophs (could grow in minimal medium).
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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
Transduction
Definition: Gene transfer from a donor to a recipient by way of a bacteriophage
Unlike transformation in which the naked DNA is transferred in transduction DNA is carried by a bacteriophage.
orIn transduction, DNA is transferred from cell to cell through the
agency of viruses
NOTE :- All phages can be transducer and not all bacteria are transducible
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
Mechanism of Transduction
Bacteriophage
Bacteriophage (phage): A virus that infects bacteria.
• Firstly Descovered in 1915 by Fredrick Twort and two years later by Felix d’Herelle.• Means bacteria eater.
• A virus that infects certain type of bacteria and replicates within them
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Phage Composition and Structure Composition
Nucleic acid Protein
• Structure (T4)– Size (80 X 100
nm)
– Head or capsid
– Tail
Core
Tail Fibers
Base Plate
Head/Capsid
Contractile Sheath
Bacteriophage - structure
Infection of Host Cells by Phages
Irreversible attachment
Base plate
• Adsorption–Tail fibers– Receptor is LPS for T4
• Nucleic acid injection
• Sheath Contraction
• DNA uptake
Transduction has been found to occur in a variety
of prokaryotes, including certain species of the
Bacteria: Desulfovibrio, Escherichia,
Pseudomonas, Rhodococcus, Rhodobacter,
Salmonella, Staphylococcus, and Xanthobacter, as
well as Methanobacterium thermoautotrophicum.
EXAMPLES OF BACTERIA
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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
Types of bacterio-phage
Virulent: capable of causing infection and eventually destruction and death of the bacterial cell. These follow the lytic cycle. e.g. T4 host E.coli.
Temperate: does not cause destruptic infection instead phage DNA is incorporated into bacterium DNA and is replicated with it and after some cycle become virulent cause lysis.
e.g. lambda phage.
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Life cycle of bacteriophage 19
Transduction There are two types of transduction:
Generalized transduction:
A DNA fragment is transferred from one bacterium to another by
a lytic bacteriophage that is now carrying donor bacterial DNA due
to an error in maturation during the lytic life cycle.
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Generalised Transduction STEPS 21
1. A lytic bacteriophage adsorbs to a susceptible bacterium.
2. The bacteriophage genome enters the bacterium. The genome directs the bacterium's metabolic machinery to manufacture bacteriophage components and enzymes
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3. Occasionally, a bacteriophage head or capsid assembles around a fragment of donor bacterium's nucleoid instead of a phage genome by mistake.
steps in Generalised Transduction (cont’d)
4. The bacteriophages are released.
steps in Generalised Transduction (cont’d) 23
5. The bacteriophage carrying the donor bacterium's DNA adsorbs to a recipient bacterium
6. The bacteriophage inserts the donor bacterium's DNA it is carrying into the recipient bacterium .
24steps in Generalised Transduction (contd)
7. The donor bacterium's DNA is exchanged for some of the recipient's DNA.
Generalized Transduction
Release of phage
Phage replication and degradation of host DNA Assembly of phages particles
Infection of recipient Homologous recombination
Infection of Donor
Potentially any donor gene can transferred
Generalized Transduction
• Specialized transduction:• A DNA fragment is transferred from one bacterium to another
by a temperate bacteriophage that is now carrying donor
bacterial DNA due to an error in spontaneous induction during
the lysogenic life cycle.• In specialized transduction the phage inserts its genome at
the specific site.
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Specialised Transduction
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1. A temperate bacteriophage adsorbs to a susceptible bacterium and injects its genome .
2. The bacteriophage inserts its genome into the bacterium's nucleoid to become a prophage.
Steps in Specialised Transduction (cont’d)
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3. Occasionally during spontaneous induction, a small piece of the donor bacterium's DNA is picked up as part of the phage's genome in place of some of the phage DNA which remains in the bacterium's nucleoid.
4. As the bacteriophage replicates, the segment of bacterial DNA replicates as part of the phage's genome. Every phage now carries that segment of bacterial DNA.
Steps in Specialised Transduction (cont’d)30
5. The bacteriophage adsorbs to a recipient bacterium and injects its genome.
6. The bacteriophage genome carrying the donor bacterial DNA inserts into the recipient bacterium's nucleoid.
Specialized TransductionLysogenic Phage
Excision of the prophage
• Replication and release of phage
• Infection of the recipient• Lysogenization of the recipient
– Homologous recombination also possible
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.
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.
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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Significance of transduction It transfers genetic material from one bacterial cell to another and
alter the genetic characteristics. For example: In specialised transduction the gal gene, a cell
lacking ability to metabolize galactose could aquire the ability . It shows the evolutionary relationship between the prophage and
host bacterial cell. Prophage can exist in a cell for a long period suggests a similar
possible mechanism for the viral origin of cancer. It provides a way to study the gene linkage.
QUESTIONS
1. In ______, the transducing particle carries only specific portions of the bacterial genome.A. generalized transductionB. generalized lysogenic phase of transductionC. specialized transductionD.restricted lysogenic phase of transduction
C . SPECIALIZED TRANSDUCTION
2. T4 is _____ type of bacteriophage.A. Lytic B. VirulentC. Only AD. Both A and B
D. BOTH A and B
3. Bacteriophages are _____A. Bacteria that attacks virusesB. viruses that attacks bacteriaC. they are free living virusesD. all of above
B. VIRUSES that attacks BACTERIA
4. Genome of bacteria can be ______
A. DNAB. RNAC. Either DNA or RNAD. BOTH DNA or RNA
C. Either DNA or RNA
5. The protein coat on virus is known as _____A. CapsomereB.CapsidC. ConjugationD. Pellicle
b. capsid
6. In a bacteria virus , its DNA is located in ____A. Tail and sheathB. HeadC. tentacles D. Head and Tail
b. head