General Concepts: Components of genomes

 

In general the viruses are made up of nucleic acids (genome), proteins (capsid), and lipids (envelope). Viral genomes can be either DNA or RNA , when once inside a host cell it directs synthesis of new viral proteins, and replication of new viral genomes. Capsid is a protein covering that surrounds and protects the viral genome. It is made up of many small subunits called as capsomeres which determine the shape of the virus. The arrangement and composition of the capsomeres varies among the virus families. Envelopes are the lipid bilayer membranes that are derived from the host cell membrane when virus “buds” out from the plasma membrane or passes through a membrane-bound organelle (such as the Golgi body or endoplasmic reticulum). The envelope contains sometimes glycoprotein (protein with carbohydrate) in the form of spikes which helps them in the attachment during the time of infection to the host cell surface (gp120 in HIV). In non-enveloped viruses, grooves present in the capsid and specific capsid proteins may bind to the cell surface receptor.

 

The most important and characteristic feature of a living organism is replication of its genetic information. The mechanism of genome replication is done with greater economy and simplicity among different viruses. Different families of viruses have their genome made of either double stranded (ds) DNA or single stranded (ss) DNA or RNA. The viruses that contain RNA genome may have either positive, negative, or mixed (ambisense) polarity. In addition, they either have single or multiple segments in their genome with linear or circular topology. Each of the above parameters have their consequences for the pathways of viral genome replication, viral gene expression, and virion assembly.

 

Among the families of viruses that infect animals and human, those containing RNA genome outnumber those containing DNA genome. This disparity is even more in case of plant viruses (no double stranded DNA virus that infect plant is known).