Virus Life Cycle: Introduction, Overview, Stages, Types, Life Cycle, FAQs

Definition Of Viruses

Viruses are minute agents of infection that can only replicate inside a host cell. The virus consists of genetic material in the form of either DNA or RNA. This genetic material is surrounded by a protein coat known as the capsid. An outer lipid envelope may be present. They differ from all other microorganisms because they are completely dependent on cellular metabolism for their growth and multiplication.

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This Story also Contains

1. Definition Of Viruses
2. Structure Of Viruses
3. Classification Of Virus
4. Viral Replication Strategies
5. Steps In The Virus Life Cycle
6. Viral Pathogenesis
7. Examples Of Viral Infections

The virus life cycles have been fundamental in understanding infection, replication, and spread of viruses in the host cell. Such information is important in developing effective antiviral drugs against viruses and vaccines and public health strategies aimed at controlling and preventing viral diseases, improving the burden on global health.

Structure Of Viruses

There's a protective coat of protein called the capsid inside that contains genetic material, either DNA or RNA. Some of them include an additional outer lipid envelope, which is derived from the host-cell membrane studded with important viral proteins to enter host cells.

Classifications of viruses are based on the type of genetic material they contain; for instance, DNA viruses have their genomes made of DNA, and similarly, RNA viruses' genomes are made of RNA. It is such basic differences that affect their replication strategies, mutation rates, and methods for treatment and vaccine development.

Classification Of Virus

Viruses are classified based on several characteristics, primarily

Type Of Genetic Material

• DNA Viruses contain DNA as genetic material. Examples include the Herpesviruses, which include the Herpes simplex virus, and the Papillomaviruses, which include the Human papillomavirus.

• Viruses with RNA as genetic material include Influenza viruses and HIV, the Human immunodeficiency virus.

Symmetry Of The Virion (Virus Particle)

• The helical viruses are those in which the capsid protein is helically arranged around the genetic material. Example: Tobacco mosaic virus.

• Icosahedral Viruses include spherical capsids whose faces are 20 equilateral triangles. Example: Adenovirus.

Presence Of An Envelope

• Enveloped Viruses: These are viruses with an outer envelope composed of lipids obtained from the membrane of the host cell. Example: Influenza virus.

• Nonenveloped Viruses: Viruses that do not possess any sort of external lipid envelope. Example: Poliovirus.

Host Range And Tissue Tropism

• Some viruses have broad host ranges, and under certain circumstances, they might infect several species, including Rabies.

• Others exhibit tissue tropism, meaning they have a preference for certain cell types of a host organism. For instance, the Hepatitis B virus has a tropism for hepatocytes or liver cells.

Viral Replication Strategies

Viral replication is the process by which viruses infect host cells to produce new particles. The details of the process differ among various viruses. It usually involves attachment of the virus to the host cell, penetration of the cell membrane, replication of the genetic material, and its assembly and release in a new virion out of the host cell.

The Lytic Cycle is a style of replication wherein the virus takes over the machinery of the host cell to replicate its genetic material and produce new virions. Eventually, this leads to the host cell's bursting, or lysis, and the new viruses are then released.

The lysogenic cycle is a process wherein the viral DNA integrates into the host cell's genome. It just lies there, replicating whenever the host cell's DNA replicates as well until it finally gets induced to start a lytic cycle. The important steps of viral replication are attachment, penetration, uncoating, replication, assembly, and release.

Steps In The Virus Life Cycle

• The viral life cycle initiates upon attachment and entry into the host cell.

• The event is characterised by binding the virus to some receptors on the cell's surface and entering the interior of the cell across the cell membrane.

• Following this would be the stage of replication of the genome and gene expression, in which the genetic material of the virus is replicated and proteins are synthesised.

• Afterwards, the assembly of new viral particles follows, responsible for packaging the replicated genetic material into new capsids.

• Finally, the discharge of the virion from the host cell takes place by cell lysis or budding, thus leading to newly reformed viruses infecting cells.

Diagram Of A virus Life Cycle

The diagram below represents the process of a virus life cycle.

Viral Pathogenesis

Viral pathogenesis is the mechanism through which viruses cause illness. It comprises direct cell damage from viral replication, immune-mediated injury due to the host response to the infection, and the action of the released viral toxins. The host immune response to viral infection is attributed to the activation of innate and adaptive immunity, such as interferons and natural killer cells, T-cells and antibodies, respectively. Due to their usual functions, they recognise and eliminate infected cells during the mechanism of host defence but offer resistance to subsequent infections.

Examples Of Viral Infections

Examples of viral infections include the following: Influenza is a viral infection that presents with seasonal flu, causing fever, cough, and body aches. HIV targets the immune network, resulting in AIDS if left untreated.