Viruses are non-cellular genetic elements that use a living cell for their replication , have an extracellular state. These are ultramicroscopic particles containing nucleic acid surrounded by protein, in some cases, other macromolecular components such as a membrane like envelope.
They are incapable of growth in artificial media as they lack independent biosynthetic machinery. Growth is possible only in living cells i.e. Plant, Animal and microbial cell. Thus, they are termed as obligate intracellular parasites.
Viruses vary considerably in size as well as in shape. They are so small that they can be viewed only under the magnification of electron microscope. The smallest viruses are about 20 nm (0.02 μm), while the large viruses’ measure about 300 nm (0.3 μm). Smallpox viruses are among the largest viruses; polio viruses are among the smallest. Due to their smaller size they can easily bypass the filtration process.
Reproduction of virus occur by replication (Process in which number of replica copies of each viral component is made and then assembled to produce progeny virus).Outside the host cell, the virus particle is also known as a virion. The virion is metabolically inert and does not grow or carry on respiratory or biosynthetic functions. However they are structurally completely mature and infectious in nature.
Viruses are the smallest known infectious agents. They are all built up of nucleic acid and protein coat(s) and some may have an outer lipoprotein envelope. They replicate in cells and may thereby lead directly to cell damage and cause disease.
Viral structure- Viral structure consists of central core of nucleic acid (Genome) surrounded by capsid, which on combination is called as nucleocapsid. Certain virus consists of outer covering known as envelope.
Certain viruses contain RNA (ribonucleic acid), while other viruses have DNA (deoxyribonucleic acid) but not both. This nucleic acid portion of the viruses is known as the genome. The nucleic acid may be single-stranded or double-stranded; it may be continuous or occur in segments; it may be linear or a closed loop.
The genome of the virus is surrounded by a protein coat known as a capsid, which is formed from a number of individual protein molecules called capsomeres. Capsomeres are arranged in a precise and highly repetitive pattern around the nucleic acid. A single type of capsomere or several chemically distinct types may make up the capsid. The combination of genome and capsid is called the viral nucleocapsid.
An envelope is a membrane like structure that encloses the nucleocapsid and is obtained from a host cell during the replication process. The envelope contains viral-specified proteins that make it unique. Number of of viruses contain envelope which are termed as Enveloped viruses. E.g. herpes simplex, chickenpox, and infectious mononucleosis. Those virus which lacks enveloped are termed as Non-enveloped virus or Naked virus.
The envelope of certain viruses is a lipid bilayer containing glycoproteins embedded in the lipid. It gives a somewhat circular appearance to the virus and does not contribute to the symmetry of the nucleocapsid. Projections from the envelope are known as spikes.
The spikes sometimes contain essential elements for attachment/adsorption of virus to the host cell. E.g. Human immunodeficiency virus (HIV), causative agent of AIDS, uses its spikes for this purpose.
The nucleocapsid of viruses are constructed according to certain symmetrical patterns. Some virus have helical symmetry (nucleocapsid is wound like a tightly coiled spiral). Rabies virus, Tobacco mosaic virus etc. has helical symmetry.
Other viruses take the shape of an icosahedron, and said to have icosahedral symmetry. In an icosahedron, the capsid is composed of 20 faces, each shaped as an equilateral triangle. Yellow fever virus, polio virus etc. have icosahedral symmetry.