Short Essay on Reproduction of Viruses

Viruses are inert outside the living cell and they have no metabolic activity of their own. They multiply only inside the living cells.

The process of multiplication in viruses is well observed in the life cycles of the bacteriophages. They have two alternate mechanisms: lytic cycle and lysogenic cycle. Lytic cycle causes death of host cell but in lysogenic cycle, host cell is not destroyed.

1. Lyric cycle it consists of 5 stages

(a) Adsorption

(b) Penetration

(c) Biosynthesis

(d) Maturation and

(e) Release

The tail plate and caudal fibers anchors the virus particle to the host cell wall at a specific site. The tip of the phage tail comes in contact with the specific receptor sites of the bacterial wall.

Within a few seconds, an opening is formed in the host cell wall at the attachment site. The elastic protein sheath of the tail now contracts driving the hollow core into the host cell. The phage DNA is then forced down the hollow core and injected into the host protoplast. The protein coat of the phage remains outside.

During the next stage which is usually of 12-22 minutes duration, no virus particle as such can be seen. The virus enters the nuclear or chromatin body of the host cell and takes over its genetic machinery. No further host DNA multiplication occurs after this but only the viral DNA is multiplied utilizing the enzymatic functions of the bacterial cell. This is the biosynthetic stage since the phage DNA is replicated in the region of nuclear or chromatin body of the host cell.

The host cell’s ribosomal machinery is used to elaborate new phage coat protein. This is also called latent period or the period of eclipse.

Thereafter, the phage particles aggregate; leave the chromatin portion of the host cell. The new phage protein immediately aggregate around them. This is the maturation phase or the vegetative stage.

The final step in the phage life cycle is the liberation phase. The phage particles disintegrate the host cells of the bacteria and get released. The phage brings about lyses or destruction of the bacterial cell and thus, called the virulent phage.

II. Lysogenic Cycle:

Here the phage does not cause destruction or lyses of its host cell. The viral DNA gets incorporated into the host cell’s genome and is replicated with it. This is called lysogeny and such viruses are called temperate viruses. This life cycle consists of the following steps.

(a) attachement of the phage particles on the host surface.

(b) Phase DNA is transferred in to the host cell

(c) Then, the phage DXA integrates with the host DNA. The viral DXA within the host DNA is called prophage.

(d) The virus can remain dormant in this state for a long time. Whenever the bacterial host divides, the phage also replicates and passes on to the next generation of bacteria.

The lysogenic viruses may become lytic spontaneously or due to external stimulation.

Economic Importance:

Viruses are obligate parasites. They are the causative agents of many kinds of plant and animal diseases. More than 400 viruses cause diseases to human beings. These diseases can be classified into 4 classes,

(i) Pneumotropic diseases which cause respiratory tract infections like influenza.

(ii) dermotropic diseases- which affect skin and subcutaneous tissues, example- Chicken pox, herpes simplex, measles etc.

(iii) Viscerotropic diseases that attack blood and visceral organs by fever, dengue etc.

(iv) Neurotropic disease that damage central nervous system by rabies, polio, encephalitis etc. Thus diseases of humans due to viruses are influenza, small pox, chicken pox, measles, hepatitis, encephalitis, cold, HIV/ AIDS, hydrophobia etc. Once the disease is established, cure is not easy. Through immunization, the diseases caused by viruses can, however, be avoided.

Similarly, many plant diseases like lesions, mosaic formations, chlorosis, necrosis, breaking of blossoms, stunting, defoliation, vein clearing and vein banding etc. are caused due to viruses.

Since the viruses are obligate parasites, the living parts of plants particularly leaves are infected by the viruses. This means that productivity of plants, particularly of crop plants are drastically affected causing enormous economic loss.