Fish Diseases and Management

5.3.3 Virus replication

Viruses replicate in the cell culture systems in variety of complex and biologically different methods. These however, follow general and well defined major steps. These include

Attachment : As a first step in the replication, the virus has to get attached to the cell surface. This is mediated by the presence of surface proteins of the virus particle and the specific receptors present on the cell surfaces. Specificity of the cell surface receptor so the viral surface proteins determine whether a virus could irreversible attach to the cell surface to initiate an infection. (Diagram)

Penetration: Once the virus is attached to the cell surface, it has to enter into the cell by crossing the plasma membrane. This is accomplished by endocytosis especially in the case of unenveloped viruses and by membrane fusion in enveloped viruses. Once in the cell, the virus is transported to the site of multiplication, either to the cytoplasm or nucleus. (dia)

Uncoating: The virus genome has to be completely released from the capsid coat for the virus replication to begin. This process is known as uncoating. (dia)

Transcription : This is the process of generating mRNA from the genomic nucleic acid. Depending on the type of nucleic acid, the process would convert DNA or RNA to mRNA polarity.

Translation : The mRNA generated in the previous step acts as template for generating the viral protein in this step.

Replication of nucleic acid : Genome of the viruses are replicated based on the type of their nucleic acid. (Diagram)

Assembly and release : The virions start to emerge when threshold levels of viral nucleic acid and protein molecule have accumulated in the infected cell. The proteins build up into nucleocapsids and later lead to packaging the genome into virus particles. Apart from self assembly in some cases, the assembly requires packaging signals to be associated with the required strand of the genome.

Release of the virus particles happens through different ways. Most of the enveloped viruses are released by budding from the cell membrane of the infected cells. Viruses also exit from the infected cells by lysis of the cell

Latency : In some of the infections, the viruses do not lead to productive infections but the virus goes to latency stage by either as a sequence integrated into the cell genome or as multiple copies of covalently closed circular DNA. The latency of infection of the virus can be activated at a later stage by a process known as induction. This could happen due to a variety of factors that the cells may be subjected to.