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Transformation, Transduction and Transfection –Gene transfer methods
#4
Conjugation
Another way of transfer of bacterial DNA between different cells is conjugation. This requires a direct cell to cell contact. The conjugation process was first described by Joshua Lederberg and Edward Tatum in 1946 after their discovery of F factor or episome of Escherichia coli cells. The F factor is very well studied conjugation system. Apart from the F factor there is numerous conjugation systems present within several bacterial systems. Often these conjugation systems carry antibiotic resistant genes and they are referred as R factor. Both F and R factors have machineries enabling them to auto-replicate thus get inherited by the daughter cells during binary fission. But additionally the conjugation process helps these plasmids to be transferred between the donor cells to a recipient one.
The bacterial conjugation involves the following steps: formation of pair after mating, synthesis of the conjugal DNA, transfer of DNA followed by maturation. The mating process is facilitated by a long structure formed between the two cells known as F or sex pilus. Though around twenty genes are responsible for a successful F pilus formation, it is mainly composed of one protein called pilin. F pilus retracts itself into the donor cell by removing the pilin protein monomers from the pilus base, thereby bringing the cells together in close proximity.
A specialized DNA replication is started after the formation of the stable mating pair. A single stranded copy of the F factor DNA is synthesized (unlike normal replication, giving rise to double stranded DNA) which is transferred into the recipient cell. Inside, the recipient cell the single stranded DNA is made double stranded thereby forming a matured double stranded plasmid. The mating pair gets broken at the end of the conjugation process with both the donor and recipient cell carrying one copy of the F factor. The daughter cells of the recipient from now on will inherit the F factor.
An F factor often contributes to the transferring of chromosomal DNA as well along with F factor genes. This happens when F factor gets integrated in chromosomal DNA and then it gets replicated along with the chromosomal DNA. Thus the daughter cell inherits it as a part of the chromosomal DNA. During further conjugation often a small part of the chromosomal DNA adjacent to the F factor also gets transferred to the recipient cell along with the F factor. In addition the F factor has the ability to transfer chromosomal DNA from the donor to the recipient cell along with itself. Since these DNA sequences encode bacterial genes, there is a possibility of recombination with the same genes present in the recipient DNA. These types of donor cells having an integrated copy of the F factor are called High frequency of recombination or Hfr strains.
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RE: Transformation, Transduction and Transfection –Gene transfer methods - by debram - 09-22-2014, 10:26 PM
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