Structure of Phage Virus with Lytic and Lysogenic Cycles (Replication)

Earlier studies were made on the phage virus (Bacteriophage). This virus infects Escherichia coli (bacteria). There are different types of phage. Mostly T phages (T for type) were studied T2 and T4 are mainly used in phage studies. The structure of T4 is studied by an electron microscope.

The volume of phage is about 1\1000 of the host (bacteria). It resembles a tadpole (frog larva). It consists of the head and tail).

Head of Phage Virus:phage virus

It has an elongated head. The Head has different shapes like pyramidal, hexagonal and prism-shaped structures. The tail is attached to the head. The double-stranded DNA molecule is present inside the head.

The tail of Phage Virus:

The tail has a complex structure. A layer of protein forms the inner tube or core of the tail. This core is enclosed in a sheath. This sheath is made up of another type of protein. A collar is present on one side of the sheath.

The other side of the sheath has an end plate. Six tail fibers are attached to the end plate. Tail fibers are structures of attachment.

Replication of Bacteriophage:

The Bacteriophage or phage virus replicates only inside the bacterial cell. Phage virus shows two types of cycles during its replication:

1. Lytic cycleLytic cycle

The phage virus causes the lysis of the bacteria in the lytic cycle. Such viruses are called lytic or virulent phages. The lytic cycle is divided into the following steps:

i. Attachment or adsorption

The Bacteriophage is attached to the receptor site on the cell wall of the bacterium. A weak chemical union takes place between the virus and the receptor site.

ii. Penetration

The tail of the virus releases lysozyme enzymes. It dissolves a small part of the bacterial cell wall. The tail sheath contracts and pushes the tail core into bacteria through its cell wall and cell membrane.

The virus injects its DNA into the bacterial cell like a syringe. The protein coat (tail and head) of the virus remains outside the bacterial cell. However, many animal viruses enter the host as a whole.

iii. Multiplication

Inside the bacterial cell, the viral DNA starts controlling the biosynthetic machinery of the bacteria. It induces the bacterial cell to synthesize the parts of the virus, protein, and DNA. The virus multiplies to form many new viruses. Approximately 200 new viruses are formed after 25 minutes of the infection.

iv. Lysis

Many viruses are formed in bacteria. The bacterial cell undergoes lysis and bursts. The newly formed viruses are released and infect other bacteria.

2. Lysogenic CycleLysogenic Cycle

The phage does not always destroy bacteria. Sometimes it lives peacefully, inside the bacterial cell. It does not control the biosynthetic machinery of the host bacteria. The viral DNA is incorporated into the bacterial chromosome.

The phage in this state is called Proghage. This process is called lysogeny. The bacteria continue to live and reproduce normally in this condition. The viral DNA becomes part of the bacterial chromosome. It passes to each daughter cell in all coming generations.

Sometimes, the viral DNA detaches from the bacterial chromosome and starts the lytic cycle. This process is called induction. The isogenic bacteria are resistant to viral infection. The phage which causes misogyny is called temperate or lysogenic phage.

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