Before we shall go for the study of structure of virus we need to have some brief discussion about the classification of virus. The classification of virus is largely in explicit due to that majority of them are still unexplained. In the year 1967 Gibbsconcluded that the inadequate knowledge about virus is the only reason that they cannot be classified in the Linnaean binomial system. Scientist Andre Lwoff included all the viruses under one phylum Vira and classified them on the basis of their physico-chemical properties. It was divided into two sub-phyla, five classes, eight orders and twenty one families and also several species.
Those virus which are always infect the plants and mainly contain RNA with few exception are called the plant virus.Those virus reproduce and induce infection in animal cells are called the animal virus. They may contain both DNA and RNA. And those viruses attack and multiply within bacterial cells are called the bacterial virus or Bactriophage. They mostly contain DNA with some exception.
Structured of virus: –
The complete virus particle consists of a nucleic acid core, the protein coat and may have an outer lipid coat occasionally. Generalised structure of virus components are as follows:
i) Envelope: -
Envelope is not present in all virus, but may be seen in some animal viruses like mumps virus, influenza virus which is called Lipo virus. They are present outside the protein coat or capsid.
Structure of envelope: It is a loose, flexible envelope formed part lipid and with great thickness of 10 to 15 nm. There are some projections called spikes, which is about 10 nm long and present in this membrane.
Functions of envelope : the envelope protect the virus particle against host’s cell wall by the production of enzymes.
ii) Capsid: -
In the structure of virus capsid is the closely fitted protein shell surrounding the nucleic acid core.
Structured of capsid: the capsid is formed of morphologically identical protein subunits called capsomeres. Every capsomeres is formed of polygonal subunits called monomers with central space of 40Å. By one or two coiled polypeptides the monomeres are formed. The capsomeres are held together by bonds and the present cuboidal or helical symmetry. The nucleic acid and capsid to kid constitute a nucleocapsid. This single capsid is absent in the virus named vaccinia, and it replaced by multiple protein coat. There may have some internal protein coats in addition to the normal protein coats.
Chemical composition: the composition of the capsid varies from virus to virus in T.M.V., the protein coat contains carbohydrate and 2.2% of other substances including some specific proteins. In Myxovirus, there have 10% of lipids and 6% of carbohydrate. But the vaccinia virus contains 1% of carbohydrate, 4% of lipids, 50% of protein, riboflavins, copper, biotin along with some enzymes and co-enzymes.
Function of capsid: capsid protects the nucleic acid against the nuclease enzymes of the host cell. It helps to Penny trait the body of host cell and in the rising of anti-viral antibodies.
iii) Nucleoid: -
The structured of virus nucleoid contains the central nucleic acid core covered by capsid. The nucleic acid is either DNA or RNA, it is linear in most of the virus, cyclic in Coliphage, circular in Lamda-phage. Unlike the higher organisms, the viruses do not contain DNA and RNA, the DNA is generally the double stranded, while RNA is single-stranded. There has some exceptions, such as single-stranded DNA in Coliphage fd, and double standard RNA in Reovirus.