Bacteriophages show two types of life cycles:
1. Lytic cycle:
This type of life cycle is seen in virulent phages such as T4 and other T-even coliphages (T2, T4). In this type of cycle, viral infection is followed by lysis (bursting and death) of the host cell and release of new infective viral particles take place.
The lytic cycle involves the following steps:
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Adsorption or Attachment: The phage attaches itself onto specific receptor sites present on the bacterial (host) cell wall.
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Digestion of the bacterial cell wall: After attachment, the cell wall of the bacterial cell is digested by lysozymes attached to the tail of the phage. Lysozyme drills a hole in the mucopeptide layer of the bacterial cell wall.
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Injection of viral DNA: The viral genetic material (DNA) penetrates into the host cell.
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Eclipse period: Synthesis of new phage DNA and viral protein coats takes place.
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Assembly: Of phage DNA into protein coats
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Lysis and Release: The bacterial cell is lysed to release the infective progeny phages.
Such as phage is termed as virulent or lytic phage since it causes death of the host cell by lysis and is also capable of causing infection.
2. Lysogenic cycle:
This type of life cycle is seen in lambda (λ) phages and certain bacteriophages such as P1. This type of cycle is known as Temperate infection.
The adsorption and injection of nucleic acid is quite similar to the lytic cycle. However, in a lysogenic cycle, the phage DNA remains in the bacterial cell and is replicated along with the bacterium. This means that whenever the lysogenic bacteria divides, each daughter cell receives at least one phage genome along with the bacterial genome.
Such an integrated and dormant viral genome is called as prophage or provirus.
This calm lysogenic period can be terminated by some shock treatment such as changes in temperature or the use of UV radiations. This sudden shock inactivates the repressor protein of the phage so that all the genes are expressed. Then the phage enters the lytic cycle. It replicates, transcribes, translates, assembles into infectious virion particles and ultimately lyses the bacterial cell and comes out.
