Life Cycle of a Bacteriophage

The bacteriophage, a virus that parasitizes a bacterium, exhibits two types of relationships with its host: the master-slave relationship known as the Lytic cycle, and the host-guest relationship known as the Lysogenic cycle.

Lytic Cycle

In the master-slave relationship, the phage virus undergoes the following stages:

a) The phage head recognizes and attaches to a specific cell surface of the bacterial host, aided by the insertion of the core into the host cell. The phage injects its genetic material into the bacterium.

b) Once inside the cell, the phage genes take control and use the host's enzymes and ribosomes to produce enzymes that replicate the phage DNA.

c) As more copies of the phage DNA accumulate, proteins form the head and tail of the phage, following the instructions of other phage genes. New phage particles are assembled within 12 to 15 minutes of infection.

d) The remaining phage genes produce an enzyme called lysozyme, which attacks the bacterial cell wall from the inside, causing the cell to rupture about 30 minutes after the phage DNA insertion. This releases new phage particles, completing the cycle and preparing for repetition. This cycle is known as the lytic cycle.

Lysogenic Cycle

In the host-guest relationship, the bacteriophage, known as a temperate bacteriophage, can infect a host cell without producing progeny viruses or damaging the host. This association is called lysogeny, or the host-guest relationship, and occurs through the following mechanism:

a) After penetration, the viral chromosome produces proteins that bind specifically to the viral DNA and inhibit replication of the viral DNA.

b) The viral DNA integrates into and becomes a part of the host chromosome. This integrated viral DNA is called a prophage.

c) The prophage replicates whenever the bacterial chromosome replicates, so that all progeny cells inherit one copy of the prophage in their chromosomes, carrying the potential to produce the temperate bacteriophage.

This cycle of the phage is known as the lysogenic cycle. Sometimes, the prophage becomes reactivated and reproduces like the lytic phase.