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Genetic Recombination By Transduction :

1 Genetic Recombination By Transduction BY: NIKHIL GABA


Contents Historical Definition Bacteriophages Life cycles Generalized Transduction Specialized Transduction Applications 2


3 historical Lederberg & Zinder Transduction was first discovered in 1952 by Joshua Lederberg and Norton Zinder Joshua Lederberg Norton Zinder Alcamo's and Prescott , google images

How transduction was discovered:

How transduction was discovered 4 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.

Unexpected discovery:

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). Alcamo's and Prescott 5

U-tube Experiment:

U-tube Experiment 6

conclusion :

conclusion Cell-to-Cell contact was not necessary for this type of recombination, and therefore it was different from conjugation. Transfer of genes with the help of Phage P22(transducers) . Named this recombination as TRANSDUCTION Alcamo's and Prescott 7


Transduction Definition Transduction involves transfer of host genes from one bacterium to another by Bacteriophages . or In transduction, DNA is transferred from cell to cell through the agency of viruses Alcamo's and Prescott 8

Slide 9:

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 M ethanobacterium thermoautotrophicum . 9 EXAMPLES OF BACTERIA

Slide 10:

10 Transfer of host genes by viruses can occur in two ways . Generalized transduction And Specialized transduction Alcamo's and Prescott


11 BACTERIOPHAGES Described 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 Alcamo's and Prescott

Structure of Viruses:

Structure of Viruses 1. Intra cellular infectious agent 2. Protein coat surrounding a nucleic acid’s core 3. Genomes : DNA or RNA (four to hundreds genes) 4. Capsids and envelopes Capsid = Protein coat that encloses the viral genome Envelope = Membrane that enclose some viral capsids Helps viruses infect their host Reproduce inside a host cell 12

Structure of Viruses:

Structure of Viruses 13 Google

Slide 14:




Examples OF PHAGES:

Examples OF PHAGES λ phage – Lysogen T2 phage T4 phage 169 to 170 kbp , 200 nm long T7 phage T12 phage R17 phage M13 phage 16


examples MS2 phage (23–25 nm in size) G4 phage P1 phage Enterobacteria phage P2 P4 phage Phi X 174 phage 17

Slide 18:

18 All phages can be transducer and not all bacteria are transducible. NOTE

Viral Multiplication Cycles:

Viral Multiplication Cycles 5 Steps Attachment Penetration Replication Assembly Release Lytic cycle Lysogenic cycle 20 Alcamo's and Prescott

DNA is penetrated:

DNA is penetrated 20 Google


21 LYTIC LIFE CYCLE Virulent phages only follow a lytic life cycle Phage directs the synthesis of many copies of phage components Primarily genetic material and coat proteins Components assemble to form new phages Bacterial host cell is lysed Newly made phages are released T 4 phage , host cell( E.coli ) Alcamo's and Prescott


MICROSCOPIC VIEW 22 Google images


CELL LYSIS 23 Google images


24 LYSOGENIC LIFE CYCLE Temperate phages generally follow a lysogenic life cycle Phage genetic material integrates into the host’s chromosome Forms a “ prophage ” Prophage exists in a dormant stage for a long time No new phages are made Do not kill the host cell Alcamo's and Prescott

Slide 25:

Prophage is copied during cell division Each daughter cell receives the prophage Λ phage , host cell(E.coli) Alcamo's and Prescott 25

Viral Multiplication Cycles:

Viral Multiplication Cycles 26

Transduction :

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. 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. Alcamo's and Prescott 27

Generalised Transduction :

Generalised Transduction 28 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

Slide 29:

29 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) :

steps in Generalised Transduction (cont’d) 30 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 .

Slide 31:

31 steps in Generalised Transduction (contd) 7. The donor bacterium's DNA is exchanged for some of the recipient's DNA .

Specialised Transduction:

Specialised Transduction 32 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):

steps in Specialised Transduction (cont’d) 33 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) :

steps in Specialised Transduction (cont’d) 34 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 .

Slide 35:

Events Leading to Lysogeny - The Prototype Phage: Lambda Circularization of the phage chromosome Lambda DNA is a double stranded linear molecule with small single stranded regions at the 5' ends . Single stranded ends are complementary ( cohesive ends ) so that they can base pair and produce a circular molecule . In the cell the free ends of the circle can be ligated to form a covalently closed circle . 35

Circularization of the phage chromosome:

Circularization of the phage chromosome 36

2. Site-specific recombination:

2. Site-specific recombination Integration site is known as att P , in E. coli the site is att B . The att sites contain the binding sites for the proteins that mediate lambda recombination. The integration reaction (att B x att P) is mediated by the proteins integrase (Int) and host integration factor (IHF). 37

Slide 38: 38 Site-specific recombination

Slide 39:

λ DNA integration 39


comparison Generalized Transduction Viruses penetrate host cell and enter lytic cycle . Viral DNA replicates immediately in bacterial cytoplasm. Specialized Transduction Viruses penetrate host cell enter lysogenic cycle . Viral DNA incorporates into the bacterial chromosome as a prophage and replicates later . 40 Alcamo's


comparison Bacterial DNA may be randomly packaged into new phage. Phage having bacterial DNA is defective. Donor bacterial genes are incorporated into chromosome of recipient bacterium. Bacterial genes and phage DNA are packaged into into new phage. Phage having both viral DNA and bacterial DNA defective. Donor bacterial genes are incorporated into chromosome of recipient bacterium together with phage DNA . 41 Alcamo's


comparison Transduction It requires a vector . Only 1-2 genes are transferred at one time. carrying of genes by a virus. Transformation Not required Many are transferred. It is absorption of foreign DNA segments from outside. 42 Trueman’s


comparison It involves bringing genes from a living .host . It can occur any time. Does not require calcium chloride. It involves absorption of gene from non living medium. Receptivity for transformation is present for a brief period at the end of active growth . Calcium chloride or some other chemical with similar property is required . 43 Trueman’s

Applications of Transduction:

Applications of Transduction Therapeutic Applications 1. Bacteria produce and secrete human insulin & human growth hormone. 2. Cells can be engineered to produce a pathogen's surface protein, which can be used as a subunit vaccine . 3. Animal viruses can be engineered to carry a gene for a pathogen's surface protein. When the virus is used as a vaccine , the host develops an immunity to the pathogen. 4 . Gene therapy can be used to cure genetic diseases by repairing the defective or missing gene. 5. spell out complete nucleotide sequence of various organisms including human beings . Trueman’s 44

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6. Diagnosis of Diseases both genetic and infectious by DNA technology. Trueman’s 45

Applications :

Applications Agricultural Applications 1. Genes for herbicide resistance (HR), insect resistance (IR; Bt or Bacillus thuringiensis toxin), and pectinase suppression (longer shelf life) have been engineered into crop plants . 2. Rhizobium has been engineered for enhanced nitrogen fixation. 3 . Bovine growth hormone is being produced by E. coli . 4 . Production of transgenic plants ( Golden Rice – rich in Vitamin A. Trueman’s 46