Article:

In this article Replication process of viruses and stages in the lytic cycle of bacteriophage are described precisely.

Replication in Viruses:

Replication of the virus can occur in two cycles.

• Lytic Cycle
• Lysogenic Cycle

1. Lytic Cycle:

The lytic cycle is a viral replication cycle involving the destruction of the host cell. In the lytic cycle, the virus infects the host cell, takes control of its cellular machinery, replicates its genetic material, assembles new viruses, and then ruptures the host cell. This cycle is characteristic of bacteriophages.

Bacteriophages:

·        Bacteriophages are those viruses that affect bacterial cells.

• Attachment:

The virus first attached itself to a specific host cell e.g. In the case of T4 phage a commonly studied bacteriophage that infects the bacterium Escherichia coli. This attachment is done by the tail fiber of the virus having an affinity with the host cell wall.

• Receptor Site:

A virus attached to that site is called a receptor site. The virus may attached by a simple mechanical force.

• Penetration:

To infect a cell a virus must enter into the cell through the plasma membrane and cell wall if present.

• Release:

• T4 Phage:

In the case of T4 phage after attachment to the host cell, the virus first releases an enzyme that weakens the spot in the cell wall of the host cell.

The virus then injects the genetic material like a hypodermic needle, pressing its sheath up against the cell and injecting its DNA into the host cell through a weak spot in the cell wall.

• Uncoating:

 The virus has an empty capsid that is outside of the host cell. Other viruses enter into their host cell intact and once inside the capsid dissolves and the genetic material is released. This process is known as uncoating.

Once the virus has infected the bacterial cell it faces resistance to the immune system of the bacterial cell.

• Replication:

Virus nucleic acid uses the host cell machinery to make large amounts of viral components both the viral genetic material (DNA or RNA) and the viral protein that comprises the structural parts of the cell.

• DNA Viruses:

In the case of DNA viruses, the DNA transcribes itself into the mRNA molecules that are then used to direct the cell’s ribosomes

 2. Lysogenic Cycle:

Ø Lysis only occurs when the condition is unfavorable.

Process of Lysogenic cycle:

In the lysogenic cycle, the viral DNA integrates into the host cell's DNA and replicates without causing immediate harm. The viral DNA that is integrated is called a prophage. The host cell replicates normally, passing the viral DNA to its daughter cells. Under certain conditions, such as stress or environmental triggers, the prophage can exit the host DNA and enter the lytic cycle, producing new viruses and lysis of the host cell. Master and Slave relationship is developed in the lytic cycle. Viruses require a specific host determined by a specific cell wall.

 Transmission of Viruses in Plants:

Following are the methods for the transmission of viruses in plants.

1. Transmission through grafting:

It is a chief method of transmission of viral diseases when some infected parts of the plant are grafted with healthy ones. The disease can be easily transmitted to the next generation.

2. Mechanical Method:

Mechanical transmission can take place by contact with healthy and infected plants. Further types of mechanical are discussed.

i)                   By air or Wind:

ii)                By rubbing the juice of Infected Plants

iii)             By using underground contact

iv)              By Agricultural Tools

3. Seed transmission:

Virus particles are either attached to the surface of the seed or enter into the reproductive organ. The former is called surface seed transmission later will be called internal seed transmission.

i)                   Attached on the surface

ii)                Enter into the floral organ

e.g. Viruses that cause mosaic are transmitted through pollen or seed.

4. Soil Transmission:

Infected viral particles present in soil. It again infects the newly growing plants. e.g. Viruses of potato mosaic are generally transmitted through soil.

5. Transmission through Fungi:

Many Fungi also transmit viruses. E.g. Necrosis of cucumber is caused by the species of Olpidium and the wheat mosaic virus is transmitted by Polymyxa graminis

6. Transmission through Insect:

Insects are the main vectors that carry the viral particle from one place to another place. They penetrate their mouth into the viral infected parts pick up the viral particles and eject them onto healthy plants.

e.g. i)    White flies transmit leaf curl of tobacco

      ii)    Grass Hooper transmit (TMV)

iii)             Aphid transmits the mosaic of barley

7. Transmission through Nematodes:

They also act as a vector for virus transmission. They transmit the virus from one plant to another plant. They mostly transmit viruses through the root system of plants.

Control Measures:

i)                   Eradication

ii)                Elimination of insects:

The spray of insecticides is used in the case of insects. E.g. Rust and smut, Barley and wheat.

iii)             Selection of seed:

Resistant varieties of plants are used in case of infected soil or insect genetic engineering. Soil analysis and surrounding area analysis are also involved in it.

iv)              By tuber Indexing:

Pot growing is done by using a healthy plant. Then grow a healthy plant.

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