Role Of Defective Phage

Role Of Defective Phage

Role Of Defective Phage
Transducting phages are ordinarily defective, and do not have a structurally or functionally normal genome. The phage does not have the space to carry its own DNA and a large piece of foreign DNA. The transducing phage is essentially a fragment of bacterial DNA enclosed in the phage coat. The bulk of transductants do not contain any detectable phage genes. This was demonstrated as follows. Bacterial cells were grown in a medium containing 5-bromouracll (which makes the DNA heavy) and nonradioactive phosphate. They were then transferred to a medium containing thymine and radioactive phosphate (32PO4).

The cells were now immediately infected with a virulent P1 mutant which blocks bacterial DNA synthesis. The transducing phages among the progeny were found to contain heavy, nonradioactive DNA. i. e. the bacterial DNA synthesized before infection. No phage DNA was detected. The bacterial DNA enclosed in the phage- =corresponds to about 2% of the E. coli chromosome. It is not known how a bacterial DNA is cut into fragments having the same size as the phage DNA. One possibility is that after bacterial DNA is enclosed in the phage capsid, the part remaining outside is degraded by nucleases.

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