Introductory Microbiology (1+1)

II.Transduction

Transduction

 
• A DNA transferred from one cell to another through the agency of virus is referred as transduction. The transduction of bacteria occurs through bacteriophages.
• Some phages are lytic, which kill the host cell during replication, called lytic cycle and virulent phages. Ex. T2 phage, some phages replicate in the bacteria without killing the host cell, called as lysogenic cycle and temperate phage. The host cell is called as lysogen. Based on the behavior of virus, transduction occurs in two way.

  1) Generalized transduction
  2) Specialized transduction.

Generalized transduction
The generalized transduction occurs due to lytic phage. Normally, the virulent phage send its DNA after absorption into host cell and DNA will order the chromosomal DNA to synthesize viral DNA and viral proteins. Then packing takesplace and virion lyse the host cell and escape to the environment. Accidentally, sometimes viral protein packed with bacterial DNA instead of viral DNA during lytic cycle, are referred as transducing particle. This can able to infect the host cell, but cannot perform the normal lytic cycle. When the transducing particle infect the recipient cell, the DNA will penetrate into cytoplasm and by the help of RecA protein get into chromosomal DNA, which leads to genetic recombination as shown in the diagram.
The frequency of transduction is 1 in 106 to 108 cells. This method of transduction is generalized one. Any portion of donor DNA can become transducing particle and become transduced and transduction takes place; this type of transduction is referred as generalized transduction.

Specialized transduction

The best studied specialized transducing phage is the phage lamboda (?) of E. coli. The location of the ? prophage in the bacterial chromosome is almost always between the bacterial genes gal and bio. Whenever the phage genome comes out of, or is excised form the bacterial chromosome, it sometimes takes with it gal or bio genes. When phages carrying gal or bio genes infect a new host, recombination with the gal or bio genes of the host can occur. It should be noted that almost all phages that carry some bacterial genes because of ‘incorrect’ excision are defective in certain viral functions because they are missing a piece of phage genetic information taken up by the bacterial genes. They cannot proceed through their entire replicative cycle, but the cell will yield phages if it is also infected with a complete phage that can code for the missing functions of the defective phages.

On the other hand, some temperate phages can able to take up a specific portion of donor DNA as their part and transduction takes place, which is referred as specialized transduction. The temperate phage (Larnbda phage) send its DNA after absorption. The penetrated viral DNA will get integrated into bacterial chromosomal DNA and persist for ever. This stage of phage is referred as prophage. The cycle is lysogenic cycle. Due to chemical or physical induction, the prophage get activated, replicated its viral DNA and protein coat followed by assembly and lysis of the cell.

 

 

Specialized Transduction-Lambda and Lambda (dgal) formation

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