- Microbiology

Virus Components:

The Capsid, or outer protein coat - many protein subunits are assembled to form a tight "shell" (capsid) inside which the nucleic acid genome lodges for protection. This shell is usually a polyhedron (usually icosahedral), spiral (helical symmetry), or it may be more complex.

The Genome (either DNA or RNA but not both) The viral genome codes for the few proteins necessary for replication. Some proteins are nonstructural, e.g.. Nucleic acid polymerases and some are structural, i.e. they become incorporated and form part of the virion.
Some viruses acquire an outer lipoprotein coat by "budding" through the host cell membranes and are thus called Enveloped viruses. The envelop contains host and viral proteins that are important for interaction with cellular components during the process of infection and replication. The host's defense mechanisms (cellular and humoral mediated responses) are directed against these viral antigenic epitopes.

The different viruses are classified based on the above components of their structure. They are broadly classified upon the type of genomic nucleic acid, e.g. DNA or RNA, and then further by the number of strands of nucleic acid (e.g. double-stranded DNA, double-stranded RNA or single-stranded RNA. Their host range is also a viral classification consideration.

Viral Classification (from Tulane U.)

Different viruses have different proteins on their surfaces which interact with specific receptors on their host cells. The specificity of the reaction between viral protein and host receptor defines and limits the host species as well as the type of cell that is infected.

The first step in a viral infection is this binding to a host protein. Changes or damage to these binding sites (e.g. by disinfectants or drugs), or blocking these sites by antibodies can render virions noninfectious.

After binding, the coat of enveloped viruses may fuse with the host cell membrane and release the virus into the host cytoplasm. Other viruses may enter the cell by a process of "endocytosis" which involves invagination of the cell membrane to form vesicles in the cell cytoplasm.
Once inside the cell, uncoating occurs. The viral genome is released from its protective capsid to enable the nucleic acid to be transported within the cell and transcribed to form new virions. Nucleic acid replication produces new viral genomes for new virions. In general, DNA viruses replicate mainly in the nucleus and RNA viruses mainly in the cytoplasm, After the new viral genomes are created, Production and assembly of viral nucleocapsid proteins to enclose the new genomes takes place.

Release of new infectious virions is the final stage of replication. This may occur as the new viruses bud from the cell surface as occurs with many enveloped viruses or when the cell lysis, or bursts as with non-enveloped viruses. In the former case, capsid proteins and nucleic acid aggregate directly adjacent to the cell membrane, and viral-coded envelope proteins, introduced into the cell membrane, concentrate in the vicinity of capsid aggregates. The membrane surrounding the nucleocapsid then bulges out and becomes "nipped off" to form the new enveloped virion. In non-enveloped viruses, disintegration or lysis of the infected cell can result in the release of intact infectious virions.

Viral structure terms: The Capsid denotes the protein shell that encloses the nucleic acid. Capsomers are groups of the Protein subunits that compose the outside of the capsid (see the different colors in the above capsids). The capsid together with its enclosed nucleic acid is called the nucleocapsid. The nucleocapsid may be enclosed in an envelope (phospholipid membrane) which may contain proteins of the host as well as of viral origin. The viron is the complete infective virus particle that can cause us trouble.

Lets look at the HIV virus life cycle.