Replication cycle

Virus Replication Cycle

The virus replication system can be classified into eight stages which includes 1) Attachment 2) Penetration 3) Uncoating 4) Gene expression 5) Genome Replication 6) Assembly 7) Maturation 8) Release

Attachment: This is the crucial stage for the initiation of virus life cycle within a living cell. As a virion comes across a specific host cell, it first binds to the host cell.
Viruses have specific proteins on their surface to attach to a host cellular surface molecule. The cellular molecules that allow the virus to attach on the cell surface are called virus receptors and the virion proteins that mediate the attachment are called as attachment proteins.
The attachment proteins and their positioning vary for different type of viruses. This protein is needed by the virus to attach to its target (host) cell before it can enter that cell.
Penetration: Penetration occurs almost instantaneously after attachment and is a next step for gaining entry into the cytoplasm by crossing the plasma membrane. The penetration process differs for different viruses.
Penetration by the process of endocytosis is common to both enveloped and non-enveloped viruses. While some virus species can directly penetrate the plasma membrane and inject their genetic material into the cytoplasm, the majority of viruses enter cells via endocytosis. Direct penetration is observed only in non-enveloped viruses, membrane fusion is observed only in enveloped viruses. Some of the non-enveloped viruses like picornaviruses and phages are capable of directly injecting their genome into the host cell.
Uncoating: Uncoating occurs simultaneously with or rapidly after penetration. Hence, this stage is difficult to be studied and remains a relatively poorly understood stage of the replication cycle.
Uncoating in general refers to the events that expose the viral genome to the host cellular machinery and sets the stage for the viral genome to express its functions required for the replication.
This stage occurs simultaneously with or rapidly after penetration. In order to express the viral genome to the cell organelles, it is necessary that the virion coat be removed partially or completely. Therefore, once virions are in the cytoplasm, they are generally uncoated to some extent by a variety of processes, including simple dissociation and/or enzyme-mediated partial degradation of the particles, to release the viral genome as a naked nucleic acid or as a nucleoprotein complex.
Synthesis: The viral synthesis stage begins immediately after the process of uncoating- the process that exposes the viral nucleoprotein complex to the cellular environment. The synthesis stage leads to the massive production of viral components.
The viral genome synthesis involves two stage Genome expression and Genome replication. Genome replication is the production of viral genomes; Genome expression is the production of viral proteins. Genome expression is a multi-step process that includes transcription and translation.
Assembly follows the viral synthesis phase. With synthesis, the host cell is full of viral genomes and proteins but the viral components are produced separately at different locations.
Therefore, after synthesis, the next task for the viral components is to assemble so that progeny viruses are formed.
The assembly stage involves the forming of a stable protein coat and packing of viral genome into the protein coat. For the successful assembly, the virus must make suitable quantities of viral genome that approximately match the number of capsids being assembled.
Maturation follows assembly. However, this is true only for non-enveloped viruses. Maturation process in general, is the stage in which the assembled premature virions become infectious viral particles. The immature virions undergo structural modifications that lead to maturation. These structural changes cause the assembled virion particles to become stable and infectious.
In the case of non-enveloped viruses Maturation is an immediate effect of assembly and are mostly inseparable whereas for some viruses like enveloped, maturation occurs with the release process or after the release process.
Release is the last stage of virus replication cycle. In this stage, thousands of newly formed virus progeny leave the host cell in search of a new host cell, thus establishing an infection in the body.
For non-enveloped viruses, the maturation process is followed by release mechanism while in enveloped viruses; the release mechanism involves assembly and maturation of the viruses.The non-enveloped viruses attain full infectivity or mature fully inside the cell and are mostly released by rupturing the cell. This process is called as cell lysis while the enveloped viruses assemble and are simultaneously released by the process of budding. Some enveloped viruses like HIV can infect the neighbouring cells and spread infection through the process of cell-cell fusion.