Viral replication

Viral replication is the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. Through the generation of abundant copies of its genome and packaging these copies, the virus continues infecting new hosts. 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.

Viruses multiply only in living cells. The host cell must provide the energy and synthetic machinery and the low-molecular-weight precursors for the synthesis of viral proteins and nucleic acids.

Virus replication occurs in seven stages:

It is the first step of viral replication. Some viruses attach to the cell membrane of the host cell and inject its DNA or RNA 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 (capsid), 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 rhinovirus uses their virus attachment protein to bind to the receptor ICAM-1on host cells that is normally used to facilitate adhesion between other host cells.

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 endocytosis, 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.

Viruses enter host cells using a variety of mechanisms, including the endocytic and non-endocytic routes. They can also fuse at the plasma membrane and can spread within the host via fusion or cell-cell fusion. Viruses attach to proteins on the host cell surface known as cellular receptors or attachment factors to aid entry. Evidence shows that viruses utilize ion channels on the host cells during viral entry. Fusion: External viral proteins promote the fusion of the virion with the plasma membrane. This forms a pore in the host membrane, and after entry, the virion becomes uncoated, and its genomic material is then transferred into the cytoplasm. Cell-to-cell fusion: Some viruses prompt specific protein expression on the surfaces of infected cells to attract uninfected cells. This interaction causes the uninfected cell to fuse with the infected cell at lower pH levels to form a multinuclear cell known as a syncytium. Endocytic routes: the process by which an intracellular vesicle is formed by membrane invagination, which results in the engulfment of extracellular and membrane-bound components, in this context, a virus. Non-endocytic routes: the process by which viral particles are released into the cell by fusion of the extracellular viral envelope and the membrane of the host cell.

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 endosomes to activate their uncoating mechanism. This involves the rhinovirus 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.

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 cytosol, and can directly access the host cell's ribosomes to manufacture viral proteins once the RNA is in a replicative form.