Replication 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.