User:Rajr2020/Viral replication

Viral replication is the formation of biological virusesLinks to an external site. during the infection process in the target host cells. The first step in viral replication is to gain entry into a host cell. Once the virus is inside the host cell, it releases and replicates its genome using resources from the host cell. During the viral genome replication, viral proteins are also being synthesized simultaneously using host cell ribosomesLinks to an external site.. Finally, the viral genomes and proteins are assembled into functioning virions that are released from the host cell. Replication between viruses is greatly varied and depends on the type of genes involved in them. Most DNA viruses assemble in the nucleus while most RNA viruses develop solely in cytoplasm.

Viral production / replication[edit]
Viruses multiply only in living cells. The hostLinks to an external site. cell must provide the energyLinks to an external site. and synthetic machinery and the low- molecular-weight precursors for the synthesis of viral proteins and nucleic acids.

The virus replication occurs in seven stages, namely;


 * 1) Attachment
 * 2) Entry (Penetration)
 * 3) Uncoating
 * 4) Replication
 * 5) Assembly
 * 6) Maturation
 * 7) Release (Liberation Stage).

Attachment[edit]
It is the first step of viral replication. Some viruses attach to the cell membraneLinks to an external site. of the host cellLinks to an external site. and inject its DNALinks to an external site. or RNALinks to an external site. into the host to initiate infection. Attachment to a host cell is often achieved by a virus attachment protein that extends from the protein shell, also known as a capsidLinks to an external site., of a virus. This protein is responsible for binding to a surface receptor on the plasma membrane (or membrane carbohydrates) of a host cell. Viruses can exploit normal cell receptor functions to allow attachment to occur by mimicking molecules that bind to host cell receptors. For example, the rhinovirusLinks to an external site. uses their virus attachment protein to bind to the receptor ICAM-1Links to an external site. on host cells that is normally used to facilitate adhesion between other host cells.

Entry (Penetration)[edit]
Entry, or penetration, is the second step in viral replication. This step is characterized by the virus passing through the plasma membrane of the host cell. The most common way a virus gains entry to the host cell is by receptor-mediated endocytosisLinks to an external site., which comes at no energy cost to the virus, only the host cell. Receptor-mediated endocytosis occurs when a molecule (in this case a virus) binds to receptor on the membrane of the cell. A series of chemical signals from this binding causes the cell to wrap the attached virus in the plasma membrane around it forming a virus-containing vesicle inside the cell.

Uncoating[edit]
Uncoating is the third step in viral replication. Uncoating is defined by the removal of the virion's protein "coat" and the release of its genetic material. This step occurs in the same area that viral transcription occurs. Different viruses have various mechanisms for uncoating. Some RNA viruses such as Rhinoviruses use the low pH in a host cell's endosomesLinks to an external site. to activate their uncoating mechanism. This involves the rhinoviruse releasing a protein that creates holes in the endosome, and allows the virus to release its genome through the holes. Many DNA viruses travel to the host cells nucleus and release their genetic material through nuclear pores.

Replication[edit]
The fourth step in the viral cycle is replication, which is defined by the rapid production of the viral genome. How a virus undergoes replication relies on the type of genetic material the virus possesses. Based on their genetic material, viruses will hijack the corresponding cellular machinery for said genetic material. Viruses that contain double-stranded DNA (dsDNA) share the same kind of genetic material as all organisms, and can therefore use the replication enzymes in the host cell nucleus to replicate the viral genome. Many RNA viruses typically replicate in the cytosolLinks to an external site., and can directly access the host cell's ribosomesLinks to an external site. to manufacture viral proteins once the RNA is in a replicative form.

Assembly[edit]
Assembly is when the newly manufactured viral proteins and genomes are gathered and put together to form immature viruses. Like the other steps, how a particular virus is assembled is dependent on what type of virus it is. Assembly can occur in the plasma membrane, cytosol, nucleus, golgi apparatus, and other locations within the host cell. Some viruses only insert their genome into a capsid once the capsid is completed, while in other viruses the will capsid will wrap around the genome as it is being copied.

Maturation[edit]
This is the final step before a competent virus is formed. This typically involves capsid modifications that are provided enzymes (host or virus-encoded).

Release (liberation stage)[edit]
The final step in viral replication is release, which is when the newly assembled and mature viruses leave the host cell. How a virus releases from the host cell is dependent on the type of virus it is. One common type of release is budding. This occurs when viruses that form their envelopeLinks to an external site. from the host's plasma membrane bend the membrane around the capsid. As the virus bends the plasma membrane it begins to wrap around the whole capsid until the virus is no longer attached to the host cell. Another common way viruses leave the host cell is through cell lysisLinks to an external site., where the viruses lyse the cell causing it to burst which releases mature viruses that were in the host cell.