Virology is the study of viruses and virus-like agents. A virus is an infectious nonliving particle that reproduces inside the cell of a living host. They are submicroscopic, parasitic particles of genetic material encased in a protein coat and sometimes further enclosed in a membrane. They are unable to multiple outside a host cell (intracellular obligate parasite). Viruses require a host cell to replicate itself and uses the host cell replication and protein synthesis machinery to create progeny of its own. They vastly outnumber their host cells.
A computer model of a Human Poliovirus
(Images from cosmosmagazine.com)
Virology is considered a subfield of microbiology or of medicine. This branch of science focuses on virus replication, pathogenesis, structure, evolution, interaction with host organism physiology and immunity, diseases they cause, virus isolation and how to culture them and their use in research and therapy. An individual who studies virology is known as a virologist.
Viruses are considered nonliving because it cannot exist purely own its own. Without a host cell, viruses cannot carry out their life-sustaining functions of reproduction. Viruses are metabolically inert and only replicate in living cells. They cannot synthesize proteins because they lack ribosomes and must use the ribosomes of their host cells to translate viral messenger RNA into proteins. They derive their energy and all other metabolic functions from the host cell as they cannot generate or store energy in the form of adenosine triphosphate (ATP). Viruses parasitize the cell for basic building materials such as amino acids, nucleotides, and lipids (fats).
Unlike still simpler infectious agents viruses contain genes which give them the ability to mutate and evolve. Over 5,000 species of viruses have been discovered
Viruses are not cellular, they do not have cells that divide and are referred to as particles with a size ranging from 20-400 nm. They have a size of roughly 1/100 of bacteria (or smaller) and are measured in nanometers. Majority of viruses cannot be seen with a light microscope because the resolution of a light microscope is limited to about 200nm. The shape and structure of the viruses have been studied using electron microscopy, nuclear magnetic resonance (NMR) microscopy, and x-ray crystallography. All viruses contain the following two components: - nucleic acid and capsid.
a) Nucleic acid/ Genome
The core of the virus is made up of nucleic acids; it can either be a Deoxyribonucleic acid (DNA) or Ribonucleic Acid (RNA) but not both. The nucleic acid (RNA/DNA) is used as templates for transcription and translation for the synthesis of proteins using host cell machinery for viral replication.
This is a protein layer/ shell that encloses the nucleic acid. Functions of the capsid are to: protect the viral genome (nucleic acid) from external environmental insults such as digestion by enzymes (e.g. nucleases) and UV –light. Also, it contains special sites on its surface that allow the virion (a single virus is in its complete form and has reached full infectivity outside of the cell) to attach to a host cell and provides proteins that help in the delivery of the viral genome into the host cells.
Some viruses have a glycoprotein envelope surrounding the nucleocapsid. It is a lipoprotein bilayer; unit membranes of two lipid layers interspersed with protein molecules and it is usually derived from the modified host cell membrane. The envelope aids in the infection process by initiating the attachment process. Some viruses develop further structures on the envelope like spikes that aid the virus during the infection process. The proteins in the envelope help to attach to the host cell surface receptors as well as fusion and entry of the viral particles into the host cell.
(Images from en.wikipedia.org)
Viruses can infect all forms of life, from microorganisms such as bacteria to highly evolved species like animals and humans.
Viral classification is usually based on phenotypic traits such as morphology, nucleic acid type, mode of replication, host organisms and type of diseases caused. Two schemes are used for virus classification:-
a) Baltimore classification
b) International committee on taxonomy of viruses (ICTV)
Virologists also study subviral particle infectious entities notably smaller than viruses. These include:-
a) Viroids- naked circular single-stranded RNA molecules infecting plants. There is no evidence that viroids encode proteins or messenger RNA (mRNA). Unlike viruses which parasitize the host translation machinery, viroids are parasites of the cellular transcription proteins: they depend on cellular RNA for replication. Viroids are not known to cause human diseases.
b) Satellites- are subviral particles that contain nucleic acid genome encoding a structural protein that encloses the satellite genome in a protein shell. They require a helper virus for infection and reproduction but the replication of the helper virus does not depend on the satellites particles.
c) Prions – are proteinaceous (consists of proteins) infectious particles that contain no nucleic acid. They are misfolded proteins which characterize several fatal neurodegenerative diseases in animals and humans. Prions induce abnormal folding of specific normal cellular proteins called prion protein ( PrP) that are found most abundantly in the brain.
The evolutionary relationships of various virus groups remain unclear.