Coronavirus

Coronavirus: One of a group of RNA viruses, so named because they look like a corona or halo when viewed under the electron microscope. The corona or halo is due to an array of surface projections on the viral envelope.

The coronavirus genome is a single strand of RNA 32 kilobases long and is the largest known RNA virus genome. Coronaviruses are also unusual in that they have the highest known frequency of recombination of any positive-strand RNA virus, promiscuously combining genetic information from different sources.

Coronaviruses are ubiquitous. They are the second leading cause of the common cold (after the rhinoviruses). Members of the coronavirus family cause major illnesses among animals, including hepatitis (inflammation of the liver) in mice and gastroenteritis (inflammation of the digestive system) in pigs, and respiratory infections (in birds).

Soon after the start of the outbreak of SARS (severe acute respiratory syndrome) in 2002-2003, a coronavirus came under suspicion as one of the leading suspects. A new coronavirus was, in fact, discovered to be the agent responsible for SARS.

The first coronavirus was isolated in 1937. It was the avian infectious bronchitis virus, which can cause devastating disease in chicken flocks. Since then, related coronaviruses have been found to infect cattle, pigs, horses, turkeys, cats, dogs, rats, and mice. The first human coronavirus was cultured in the 1960s from nasal cavities of people with the common cold. Two human coronaviruses, OC43 and 229E, cause about 30% of common colds. The SARS coronavirus is different and distinct from them and from all other known coronaviruses.

Coronaviruses are very unusual viruses. They have a genome of over 30,000 nucleotides and so are gigantic, as viruses go. They are also unusual in how they replicate themselves. Coronaviruses have a two-step replication mechanism. (Many RNA virus genomes contain a single, large gene that is translated by the cellular machinery of the host to produce all viral proteins.) Coronaviruses can contain up to 10 separate genes. Most ribosomes translate the biggest one of these genes, called replicase, which by itself is twice the size of many other RNA viral genomes. The replicase gene produces a series of enzymes that use the rest of the genome as a template to produce a set of smaller, overlapping messenger RNA molecules, which are then translated into the so-called structural proteins — the building blocks of new viral particles.