Multiplication of Viruses: Functions, Reproduction and Stages

Why are viruses considered nonliving when present outside the host body? Is the multiplication of viruses similar to bacterial multiplication? Viruses are obligate intracellular pathogens, meaning they cannot multiply without the help of a host cell’s machinery. Although the replicative life cycle of viruses varies significantly depending on the species and type of virus, there are six fundamental phases that all viruses must go through in order to replicate. What are those six phases? To know more about the multiplication of viruses, scroll down the article.

What are Viruses?

Viruses are small obligate intracellular parasites. They must infect and take over a host cell machinery in order to replicate. They lack the chemical machinery for generating energy and synthesising large molecules. Viruses, therefore, must find an appropriate eukaryotic or prokaryotic cell in which they can replicate.

Characteristics of Viruses

Some of the general characteristics of viruses are as follows:

i. Viruses are ultramicroscopic particles that contain nucleic acid surrounded by protein.
ii. Viruses are typically much smaller than bacteria, with the bulk of them being submicroscopic. While most viruses are 5 to 300 nanometers (nm) in size, large viruses such as Mimiviruses and Pandoraviruses with a diameter of 0.4 micrometres have been discovered in recent years.
iii. The protein coat of a virus particle is known as a capsid that gives shape or symmetry to the viruses.
iv. The capsid is subdivided into individual protein subunits known as capsomeres.
v. The capsid with its enclosed genome is known as the nucleocapsid.
vi. The nucleocapsid may be naked or enveloped. Viruses composed of only nucleocapsids are called naked viruses, and the ones surrounded by a flexible membrane called an envelope and are referred to as an enveloped virus.
vii. In certain viruses like HIV, other macromolecular components such as a membrane-like envelope are also present along with nucleic acid and proteins.
viii. Many enveloped viruses contain spikes projecting from the envelope, and these help in attachment and host cell penetration during multiplication.

Replication of Viruses

i. The production of viruses by the use of the cell machinery of the target host cells after it has been infected is known as viral replication.
ii. Before viruses can replicate, they must first enter inside the cell.
iii. The virus continues to infect new hosts by producing many copies of its genome and packaging these copies.
iv. The type of genes involved in viral replication varies significantly.
v. The majority of DNA viruses assemble in the nucleus, whereas the majority of RNA viruses develop exclusively in the cytoplasm.
vi. Many bacteriophages (viruses that infect bacteria) incorporate their genetic material into the genome of a host cell, i.e., bacterium and are capable of producing phages if specifically activated.

What are the Stages of Viral Replication?

Although the specific detail of virus replication varies from one virus to another, general replication is the same for most viruses. The stages are as follows:

Attachment

i. This is the first step in virus infection in which the interaction of virion with a specific receptor site or region on the surface of the host cell takes place.
ii. This specificity determines the host range (tropism) of a virus.
iii. The attachment of the virus particle to the receptors of the host cells primarily shows the Lock and Key model mechanism, thus rendering specificity to their infection.
iv. The virus attaches to typical cell surface components of the host cell, such as protein, polysaccharides, or the lipoprotein-polysaccharide complex.
v. For e.g., HIV binds to the CD4 cell receptor of T-lymphocytes.
vi. As an example, Pili and flagella are receptors in some bacteriophages, whereas transport binding protein and other viruses may have receptor sites.
vii. The influenza virus receptor is a glycoprotein present in RBC and other mucus membrane cells of susceptible hosts.

Penetration

i. Penetration follows attachment.
ii. The fusing of viral and cellular membranes can occur as a result of conformational changes in viral capsid proteins or the lipid envelope during the process of attachment to a particular receptor.
iii. Endocytosis or membrane fusion allow virions to enter the host cell. This is referred to as a viral entry.

Uncoating

i. Uncoating of the virus occurs shortly after the penetration process.
ii. Uncoating is the process of releasing the viral genome from the capsid so that enzymes may translate, transcribe, and replicate it.
iii. Transcription of the viral genome is usually the next step in all viruses except in those viruses whose genome behaves directly as mRNA, for example, Picornavirus.
iv. RNA viruses that carry minus (-) stranded RNA must first convert their DNA to plus (+) stranded RNA, which serves as mRNA.
v. Viral RNA polymerase, which is released after uncoating, catalyses transcription.
vi. In bacteriophages, this step does not occur.

Biosynthesis

The biosynthesis process of virus replication can be divided into two events, i.e., early events and late events.
a. Early event:
i. In most viruses, just a portion of the nucleic acid gets transcribed into mRNA at first.
ii. Early mRNA codes for essential proteins (enzymes) for nucleic acid replication.
iii. Many copies of progeny nucleic acids develop after nucleic acid replication.
b. Late event:
i. The progeny genome is used to transcribing late mRNA.
ii. For the production of viral genomic RNA and mRNA, RNA viruses often employ the RNA core as a template.
iii. The viral mRNA directs the host cell to synthesise viral enzymes and capsid proteins and to assemble new virions.
iv. Even if the mRNA is generated in the nucleus, the translation process always takes place in the cytoplasm of the host cell.
v. There are, of course, exceptions to this rule. If a host cell lacks the enzymes required for viral replication, viral genes offer the information needed to drive the production of the missing proteins.
vi. HIV and other retroviruses have an RNA genome that must be reverse transcribed into DNA before being integrated into the genome of the host cell.

Assembly

i. When a sufficient number of viral components have been produced, the mature virus is assembled.
ii. Depending on the kind of virus, assembly takes place in the nucleus or cytoplasm of the host cell.
iii. Except for Poxvirus, DNA viruses are built in the nucleus, while RNA viruses are assembled in the cytoplasm, with the exception of the Influenza virus and Reovirus.

Release

i. The ultimate step in viral replication is the release of the mature virus from the host cell.
ii. The mechanism of virus release varies with types of viruses. Cell lysis is the most common method for releasing naked viruses.
iii. The enveloped viruses are released by budding through a particular area of the host cell membrane, and the vision takes on a portion of the host cell membrane during this process.
iv. Some viral proteins stay within the host’s cell membrane after virion release, making them potential targets for circulating antibodies.
v. Residual viral proteins in the host cell’s cytoplasm can be processed and displayed on MHC class I molecules at the cell surface, where they are recognised by T lymphocytes.
vi. Some animal and plant viruses do not kill their hosts and instead disseminate the virus through specific routes.

Outcome of Virus Replication

Virus replication of host cells can have the following three possible outcomes.
i. Productive infection: The provirus is translated into messenger RNA in a productive infection, which creates a new virus that then infects additional cells via the lytic cycle. When the provirus is transcriptionally quiet rather than active, it causes a latent infection. Virions are the whole infectious virus that is produced during infection.
ii. Abortive infection: Abortive infection occurs when a virus replicates in a non-permissive host cell or when viral replication generates viral offspring that are incapable of infecting further host cells.
iii. Restrictive infection: Infective viral progeny are created at times when a host cell is transiently receptive, while the virus persists within the cell without creating infective viral progeny at other times. In rare cases, it results in viral release but no cell death.

Life Cycle of Viruses

Some of the viruses can multiply by two alternative mechanisms, i.e., the lytic cycle or the lysogenic cycle.

i. Lytic Cycle: The lytic cycle ends with the lysis and death of the host cell. For example, the bacteriophage takes over the cell, reproduces more phages, and kills the cell during the lytic cycle of virulent phage. The HIV virus is also an example of a virus multiplying through a lytic cycle.

ii. Lysogenic Cycle: The lysogenic cycle, also known as temperate or non-virulent infection, does not destroy the host cell and instead uses it as a temporary shelter where it is in a dormant state. Following the injection of phage DNA into the host cell, it uses phage-encoded integrases to integrate itself into the host genome, resulting in a prophage. The prophage genome is then copied passively alongside the host genome for as long as the host cell divides without producing the proteins necessary to generate offspring. As the phage genome is generally relatively small, the bacterial hosts are usually unaffected by this mechanism. A prime example of a phage with this type of life cycle is the lambda phage of E.coli.

Fig: Lytic and Lysogenic Cycle

Summary

Through this article, we understood that for reproduction and metabolic activities, all viruses rely on cells. Viruses do not encode all of the enzymes required for viral replication on their own. A virus, on the other hand, may steal cellular machinery to create additional viral particles within a host cell.

Although the life cycle of viruses varies significantly depending on the species, there are six fundamental stages: attachment, penetration, uncoating, biosynthesis, assembly and release. The multiplication of viruses or viral replication can be classified into two types based on life cycle, i.e., lytic cycle or lysogenic cycle.

FAQs

Q.1. Do viruses have DNA?
Ans: Viruses have either DNA or RNA as their genetic material.

Q.2. What are the two types of virus replication?
Ans: There are two types of methods used by viruses to replicate, i.e., the lytic cycle and lysogenic cycle.

Q.3. Can Covid multiply?
Ans: Covid virus cannot reproduce on its own. It can only replicate inside the living host cells, such as a human.

Q.4. What do you mean by replication of a virus?
Ans: The production of viruses by the use of the cell machinery of the target host cells after it has been infected is known as viral replication.

Q.5. What are the 6 stages of viral replication?
Ans: Attachment, penetration, uncoating, biosynthesis, assembly, and release are the six phases in the virus replication cycle that most productive viral infections follow.

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