Replication and Transmission of Viruses
Replication of Viruses:- During the process of viral replication, a virus induces a living host cell to synthesize the essential components for the synthesis of new viral particles. The particles are then assembled into the correct structure, and the newly formed virions escape from the cell to infect other cells.
1. Attachment:-
> The first step in the replication process is attachment.
> In this step, the virus adsorbs to a susceptible host cell.
> High specificity exists between virus and cell, and the envelope spikes may unite with cell surface receptors.
> Receptors may exist on bacterial pili or flagella or on the host cell membrane.
2. Penetration:-
> The next step is penetration of the virus or the viral genome into the cell.
> This step may occur by phagocytosis; or the envelope of the virus may blend with the cell membrane; or the virus may “inject” its genome into the host cell.
> The latter situation occurs with the bacteriophage when the tail of the phage unites with the bacterial cell wall and enzymes open a hole in the wall. The DNA of the phage penetrates through this hole.
3. Replication:-
> The replication steps of the process occur next.
> The protein capsid is stripped away from the genome, and the genome is freed in the cell cytoplasm.
> If the genome consists of RNA, the genome acts as a messenger RNA molecule and provides the genetic codes for the synthesis of enzymes.
> The enzymes are used for the synthesis of viral genomes and capsomeres and the assembly of these components into new viruses.
> If the viral genome consists of DNA, it provides the genetic code for the synthesis of messenger RNA molecules, and the process proceeds.
>In some cases, such as in HIV infection (as discussed below), the RNA of the virus serves as a template for the synthesis of a DNA molecule. The enzyme reverse transcriptase catalyzes the DNA's production. The DNA molecule then remains as part of the host cell's chromosome for an unspecified period. From this location, it encodes messenger RNA molecules for the synthesis of enzymes and viral components.
4. Assembly:-
> Once the viral genomes and capsomeres have been synthesized, they are assembled to form new virions. This assembly may take place in the cytoplasm or in the nucleus of the host cell.
> After the assembly is complete, the virions are ready to be released into the environment.
5. Release:-
>For the release of new viral particles, any of a number of processes may occur. For example, the host cell may be “biochemically exhausted,” and it may disintegrate, thereby releasing the virions.
> For enveloped viruses, the nucleocapsids move toward the membrane of the host cell, where they force themselves through that membrane in a process called budding. During budding, a portion of cell membrane pinches off and surrounds the nucleocapsid as an envelope.
Lytic Cycle and Lysogeny:-
Lytic cycle:-
- The replication process in which the host cell experiences death is called the lytic cycle of reproduction.
- The viruses so produced are free to infect and replicate in other host cells in the area.
Lysogeny:-
- Not all viruses multiply by the lytic cycle of reproduction.
- Certain viruses remain active within their host cells for a long period without replicating.
- This cycle is called the lysogenic cycle.
- The viruses are called temperate viruses, or proviruses, because they do not bring death to the host cell immediately.
- In lysogeny, the temperate virus exists in a latent form within the host cell and is usually integrated into the chromosome.
- Bacteriophages that remain latent within their bacterial host cell are called prophages. This process is a key element in the recombination process known as transduction.
- An example of lysogeny occurs in HIV infection. In this case, the human immunodeficiency virus remains latent within the host T-lymphocyte.
Transmission of Viruses:-
1. Seed Transmission of Virus:-
> Transmission through the seeds of the host plant was earlier considered to play a minor part in the spread of virus diseases.
> Recently Bennett (1969) listed 53 viruses which are transmitted by seeds of about 124 plant species.
> The seeds are important in the spread of a few viruses of legumes, wild cucumber, tomatoes, and curly top virus of beet sugar.
> In the latter case the seeds carry a high percentage of the virus.
> The virus, however, does not enter the embryo. It is carried in a portion of the seed of the diseased plants.
2. Transmission by Vegetative Propagation:-
> It is one of the chief methods of transmission of virus diseases especially of Potato, Rose, Sugarcane, Raspberry, Strawberry, Turnips, Bulb plants, fruit trees and many ornamentals.
> The vegetative parts, the infected plants such as the tubers, bulbs, roots, offshoots, buds and scions which are used for propagation, will contain the virus present in the parent.
> The new plants raised by the above-mentioned vegetative methods are nearly always infected.
3. Transmission by Mechanical Means:-
Many mosaic viruses are transmitted mechanically from diseased plants to healthy ones by the following methods:
(i) By contact of infected and healthy leaves brought about by wind.
(ii) By rubbing the juice of the diseased plants over the surface of the leaves of healthy plants.
(iii) By grafting infected buds on to healthy plants.
(iv) Agricultural implements also play quite an important part. The knife used for cutting the seed pieces and the pruning shears will spread the disease.
(v) Some viruses spread below ground by contact between the roots of diseased and healthy plants.
(vi) Handling plants at planting time and in cultural operation will also help in the spread of viruses such as Sugar beet. Curly top virus and Cucumber mosaic virus.
4. Transmission by Cuscuta:- In many cases Dodder (Cuscuta) serves as a transmitting agent and an effective bridge between the infected host and the healthy plants by establishing intimate biological contact through its haustoria.
5. Soil Transmission:-
> Quite a number of viruses are transmitted through the soil.
> Common examples of soil borne viruses are Potato mosaic virus, Oat mosaic, Wheat mosaic, etc.
> In all these cases the disease is contracted from the soil.
6. Insect Transmission:-
> Some plant and animal viruses are spread and complete particles introduced into host cells by arthropod vectors and even by dog-bite as in rabies.
> Among the arthropods most important agents of spread of virus diseases are the insects.
> The insect which carries the disease is called a vector.
> The insect vectors which play a major role in the dissemination of plant viruses are the Aphids, Leafhoppers, Flee beetles, Scale insects, thirps and White flies.
> Most of the insect vectors are sucking insects.
> Aphids transmit more plant viruses than any other insects. Leafhoppers come next in the list.
> About three hundred plant virus diseases are known to have insert vectors.
> The insert obtains virus through its mouth parts at the time of feeding on the diseased plant. It is then inoculated in the healthy plant by means of the mouth part. Inoculation in many cases must be in a certain tissue or upon young leaves.
> The virus may remain active in the body of the vector for many days. Instances are however, known when infectivity is soon lost. There are also cases where a vector cannot infect a healthy plant immediately after it has fed on a diseased plant.
> There is delay in the development of infective power within the vector. This period of development of infectivity for the virus within the vector is called the incubation period. The duration of the incubation period varies with different viruses from a few hours to days.
7. Transmission by Fungi:-
> The first proof of the fungus as a vector of plant viruses was found by Gorgon in 1958.
> Fie found that the diseased lettuce was invariably infected by a soil chytrid, Olpidium.
> Later he discovered that the fungus acts as a reservoir and vector of the big vein virus.
> The virus acquired by the fungus remains in the oospore. The latter germinates and produces the zoospores which function as infective agents and penetrate lettuce roots.
> Similarly tobacco necrosis virus has been reported by Teakle (1960) to enter roots of its host by the zoospores of O. brassicae.
8. Some Soil Inhabiting Viruses have Nematode Vectors:-
> Animal viruses may gain access to the higher animals through the mouth and nose from dust or contaminated food.
> Besides infection from outside, virus may also be transmitted from cell to cell but the internal transmission need not be in the form of virus particles.