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- 2. Lederberg and Tatum (1946) discovered conjugation in e.coli Detailed structure studies were made by Woodman and Jacob (1956) It was defined as process by which genetic material is transferred from one bacterial cell (“donor cell or male cell”) to another (“recipient cell or female cell”) through a specialized intercellular connection called Sex pilus or Conjugation tube. Conjugation
- 3. The maleness and femaleness of bacterial cells are determined by presence or absence of F- plasmid also called f- factor or sex- factor. F-plasmid , an extra chromosomal genetic material is always present in cytoplasm of donor or male cell. And the later develop specialised cell surface appendages called “F- pili” or “sex pili” under the control of f- plasmid where , in recipient or female cells always lack f-plasmid .
- 5. F-plasmid can exist in two different states:- The autonomous state in which it lies free in cytoplasm and replicate independent of bacterial chromosome (DNA): a donor or male cell containing f-factor in autonomous state is called F+ cell. The integrated state in which it is integrated into the bacterial chromosome (DNA) and replicates along with it ; a donor or male cell containing f-factor in integrated state is called Hfr cell or male cell, however the recipient or female cell lacks f-factor and this is called F- cell.
- 6. Conjugation between f+ cell & f- cell Step 1: F+ cell produce hair like appendages called sex pili which facilitates cell to cell contact with f-strain by forming conjugation tube. Step 2: Replication of F factor making copy. Step 3: Transfer of copy of F plasmid to recipient cell via conjugation tube. Step 4: Conjugation tube dissolves Now the f- stain is also f+ but, here bacterial genome is not involved. f+ cell + f-cell= f+cell + f+cell
- 8. Conjugation between Hfr cell and f-cell :- Step 1:-Hfr donor cell produce hair like appendages called sex pili which facilitates cell to cell contact with f- strain by forming conjugation tube. Step 2:- Hfr undergoes replication by rolling circle mechanism. Step 3:- Circular Hfr DNA breaks open and become linear. A portion of F plasmid occupies the leading end. Step 4:- The leading end moves to recipient cell via conjugation tube. Step 5:- During this transfer, conjugation tube abruptly breaks up , so that only part of Hfr DNA is transferred to the recipient.
- 9. Step 6:- The transferred donor DNA gets integrated to the recipient chromosome. This bring about genetic variability
- 10. Conjugation between f’cell & f-cell:- It is also called sex duction. Transfer of f’ factor to receipt occur in mating between f’ and f- . The transfer of f’ to recipient produce partial diploids . Recombination occur between recipient chromosome and f’, producing recombinant. Recombination of this type mediated as sex duction
- 11. TRANSDUCTION
- 12. Transduction was first discovered in 1952 by Joshua Lederberg and Norton Zinder. Studied in Salmonella typhimurium It is special method of genetic recombination where genetic material is transferred from the donor to recipient cell through a non- replicating bacteriophage- temperate bacteriophage. In this process, a small fragment of bacterial DNA is incorporated into an attacking bacteriophage and when this bacteriophage infects a new bacterial cell, it transfers the genetic material into it, and thus genetic recombination takes place.
- 13. Transduction are of two types:- Specialised transduction Generalised transduction
- 14. Specialised transduction In this process , DNA fragment is transferred from one bacterium to another bacterium by temperate bacteriophage that is now carrying donor bacterial DNA due to an error in spontaneous induction during lysogenic cycle.
- 15. STEPS INVOLVED IN SPECIALISED TRANSDUCTION 1. A temperate bacteriophage infect susceptible bacterium into bacterium. 2. The DNA of bacteriophage, incorporated into genome of bacterium called PROPHASE. 3. Occasionally, during spontaneous induction, a small piece of donor bacterial DNA is picked up as part of the phage genome. 4. As bacteriophage replicate, the replicates the segment of bacterial genome also replicate along with phage genome. 5. When this phage infect another bacterium, the donor DNA along with phage genome inserted into a recipient bacterial genome form recombinant DNA.
- 18. Generalised 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.
- 19. Steps involved in generalised transduction 1) A lytic bacteriophages infect a susceptible bacterium. 2) The phage genome enters into the bacterium. The genome directs a bacterium’s metabolic machinery to manufacture bacteriophage components and enzyme. 3) Occasionally, a bacterial genome get into a phage coat by mistake and the bacteriophage released. 4) When this phage having bacterial DNA infect another bacterium, the previous bacterial DNA incorporated into new bacterial genome forming recombinant DNA.
- 22. Significance of transduction It transfer genetic material from one cell to another cell and alter genetic characteristics. For example , In specialized transduction a cell lacking ability to metabolize galactose could acquired ability. It shows evolutionary relationship between the prophage and host bacterial cell Prophage can exist in a cell for long period suggest a possible mechanism for viral origin of cancer. It provide a way to study the gene linkage.