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The bacteriophage replicates only inside the bacterial cell. There are many steps in replication.
Attachment (Adsorption) of Phage to the Host Cell:
First of all the bacteriophage attaches to the bacterial cell at receptor site. The receptor sites are present on the cell wall of bacterium.
During attachment, week chemical union occurs between virion and the receptor site.
Penetration:
In this step the tail releases the enzyme Iysozyme. This enzyme dissolves a portion of the bacterial cell wall.
The tail sheath contracts and tail core is forced into the cell through cell wall and cell membrane.
The virus injects its DNA into the cell (just as the syringe is used to inject the vaccine).
The protein coat, consisting of head and tail, remains outside the cell.
Many animal viruses enter the host cell as a whole.
After penetration one of the following cycles take place:
(i) Lytic cycle
(ii) Lysogenic cycle
Lytic Cycle:
During lytic cycle following steps occur:
Multiplication: Soon after entering the bacterium, the viral DNA takes the control of the biosynthetic machinery of the host.
The host is forced to synthesize viral DNA and proteins. As a result viruses begin to multiply.
Within 25 minutes about 200 new bacteriophages are formed.
Lysis: After the formation of bacteriophages, the bacterial cell bursts (lysis occurs).
Newly formed bacteriophages are released to infect other bacteria. A new lytic cycle may start.
The phage that causes lysis of the host cell is called lytic or virulent phage.
Lysogenic Cycle:
In some cases instead of lytic cycle, lysogenic cycle takes place. It occurs as follows:
Formation of Prophage: The viral DNA does not take over the control of host’s machinery.
The DNA is incorporated into the bacterial chromosome. Phage at this state is called prophage and this process is known as lysogeny.
The phage which causes lysogeny is called temperate (lysogenic) phage.
Lysogenic bacteria are resistant to infection by the same or related phages.
Replication: During lysogeny the bacterium lives and reproduces normally.
Viral DNA is the part of bacterial chromosome and passes to each daughter cell generation after generation.
Induction: Sometimes the viral DNA detaches from the chromosome of the host and lytic cycle starts. This process is called induction. Induction is spontaneous or environmentally induced excision of the bacterial chromosome.