When a lytic bacteriophage infects a susceptible bacterium, it injects its DNA inside the cell. Following this, the phage DNA initiates its own replication begining with DNA replication and followed by sysnthesis of phage capsid components. At the same time, the host (bacterial) chromosome suffers damage and gets broken down to smaller pieces.
Normally, while packaging, phage DNA is incorporated inside phage capsid however on occasion chromosome DNA (roughly of same size)gets incorporated inside the phage. Following lysis of the infected all the phages are released, with few containing bacterial DNA.
When those phages that have chromosomal DNA instead of its own DNA infect another susceptible bacterium, they introduce bacterial DNA.
Such DNA recombines with bacterial chromosome at the region of homology and result in exchange of genes via recombination, thus a property originally possessed by one bacterium is introduced into another bacterium.
See flash video for understanding:
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