Transduction Feb 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