Virus , STRUCTURE OF VIRUSES, Component of virus
A virus is a small infectious agent that typically consists of a segment of nucleic acid with a protein or lipoprotein coat. Viruses replicate only inside the living cells of other organisms as they require host resources for their replication. They can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.
STRUCTURE OF VIRUSES
Viruses usually range between 10-400 nm in size while few are exceptionally large. They can be observed only by electron microscopy and X-ray crystallography. A fully assembled infectious virus is called a virion. The simplest virions consist of two basic components - a core portion containing nucleic acid (either DNA or RNA) and a protein coat called the capsid. A capsid consists of numerous subunits called capsomeres. Viruses are grouped on the basis of size and shape, chemical composition, type of genome and mode of replication.
- Helical or Cylindrical Symmetry
Helical morphology is seen in many filamentous and pleomorphic viruses. The capsomeres and nucleic acids are wined together to form a helical or spiral tube-like structure. The rod-shaped helical capsid of Capsid these viruses consists of numerous identical capsomeres wrapped around the helical filament, e.g., Tobacco Mosaic Virus.
2. icosahedral Symmetry
Many viruses appear as spherical, cuboidal or polygonal in shape which is actually icosahedral. An icosahedron is a polyhedron that has 20 equilateral triangular faces and 12 vertices. An icosahedral capsid comprises of both pentamers (pentagonal capsomeres at the vertices) and hexamers (hexagonal capsomeres at the vertices). The icosahedral capsid is the most efficient way to enclose a space. The total number of capsomeres of different icosahedral viruses varies greatly, e.g., Turnip Yellow Mosaic Virus (TYMV) has 32 capsomeres, Papilloma Virus has 72 capsomeres, Adenovirus have 252 capsomeres, etc.
3.Complex Symmetry Head
Although most of the viruses have either icosahedral or helical capsids, many viruses do not fit into either of the two categories, they combine both polygonal and filamentous shapes. E.g., Poxviruses, Bacteriophages. T4, bacteriophage comprises of the polygonal head which contains DNA genome and rod-shaped tail of long fibers.
Components of Virus
Four components of viruses are
(a) Nucleoid: Viral genome is made of a single molecule of nucleic acid. It may be linear or circular with
various degrees of coiling. The nucleic acid is the infective part of the virus. The nucleic acid is either DNA or RNA but never both. Viruses have all four possible nucleic acid types - single-stranded DNA, double-stranded DNA, single-stranded RNA, and double-stranded RNA.
(i) Double-stranded or dsDNA occurs in T, , Tg bacteriophages, Coliphage Lambda, Pox Virus, Adenovirus, etc.
(ii) Single-stranded or ssDNA occurs in Coliphage @ x 174, Coliphage fd, etc. The single strand of DNA is called a plus strand. A complementary or negative DNA strand is synthesized to produce DNA duplex for replication.
(iii) Double-stranded or dsRNA is found in Reovirus and Tumour Virus. Both are linear type.
(iv) Single-stranded or ssRNA is more common in riboviruses. The single stranded RNA is generally linear, @.9., Poliomyelitis virus, influenza virus, etc, Retroviruses have two copies of ssRNA, @g., HIV, Rous Sarcoma Virus of the mouse.
(b) Capsid (Sheath, Coat): It is the proteinaceous covering around the virus which protects the nucleoid from damage by physical and chemical agents. The capsid consists of a number of subunits called capsomeres or capsomers. The capsid of TMV has 2130 capsomeres. In binal bacteriophages, the capsid sheath of the tail is contractile.
(c) Envelope: Some animal viruses, a few plant, and bacterial viruses are bounded by an outer loose membranous layer called an envelope. In contrast to enveloped viruses, the viruses without an envelope are called naked. The envelope consists of proteins (from virus), lipids and carbohydrates (from the host). It has subunits called telomeres or peplomers. The surface of the envelope can be smooth or have outgrowths called spikes. Common enveloped viruses are HIV, Herpes Virus, Vaccinia Virus, etc.
(d) Enzymes: They are occasional. Enzyme lysozyme is present in the region that comes in contact with host cell in bacteriophages. Other enzymes are neuraminidase (in Influenza Virus), RNA polymerase, RNA transcriptase, reverse transcriptase, etc In some cases, enzymes are associated with the envelope or capsid but most viral enzymes are located within the capsid.