what is Viruses? advantages and disadvantages of Viruses? classification of viruses? types of Viruses?

Characteristics of viruses:-

The are not free living in and occur only as obligate intracellular parasites.
They have only a single kind of nucleiacid-either DNA or RNA. But never both.
Nucleic acid single molecule that have one replicon which may be single {or} double standard.
The Outer shell capsid is mostly of protein, except few animals viruses where additional polysaccharides are also present.
They can be propagated in living culture media only.
They contain no Metabolic enzymes {or} protein synthetic machinery of their own. The use host machinery for synthesis of their proteins.
They replicate, they do not grow but their nucleic acid directs the host cell to make various part of viruses and then to assemble the parts in to complete infectious particles virus.
Classification of Viruses based on nucleic acid :-
The viruses have DNA {or} RNA as genetic material. The viruses which have DNA are :-
The  polyoma virus smaller than herpes virus with fewer capsomers.
The shell is composed of 42 capsomers,12 pentagonal prism at the corner of an icosahedral and 30 hexagonal prism on 30 edges.
The 20 faces have no Capsomeres  of their own, which helps to explain nearly spherical appearance of virus.
High resolution electron micrographs revealed that structures originally identified as Capsomeres are indeed composed of still smaller subunits.
The viruses Ø×174 have been  intensely studied because it contains an unusual single stranded from of DNA. When first examined under electronic microscope, the virus appeared to have a shell of 12 spherical capsomere is an icosaheran.
Later it was shown that each Capsomere is formed from 5 subunits, but since each Capsomere may be shared with neighbour the total number of subunits is 30. If they are not shared and each capsomere have 5 subunits, arranged in a ring like structure that total would be 60 and shape of the particle would be that of a dodecahedron {12 faced  polyhedron}

Tobacco mosaic virus:-

Tmv is a rod shaped virus, each particle is elongated cigarette like in form.
Each Rod about 3000A0 Long and 150A0 diameter.
In each  Rod, there are about 2130 protein subunits the Capsomeres.
The Capsomeres are closely packed and arranged in a helical manner around the RNA helix, forming a hollow cylinder.

tobacco mosaic virus, tmv,

 There is a hallow core of about 4 nm diameter which run the entire length of the rod and it contains the RNA
The RNA does not occupy the hole but is deeply embedded in the protein subunits, does having it on helix.
RNA single stranded molecule in the form of a long helix extending the entire length of the particle.
The protein amounts about 95% of the total substances of the rod and the remaining 5% is RNA.
The proteins are of high molecular weight of about 40 million where as the  molecular weight of the RNA molecule is about 2 million.
RNA provides a code direct the synthesis of specific viral proteins in the host cell.
There are about 161/3 protein subunits in each helical turn and of the helix contain about 49 capsomeres.
Each capsomere has a molecular weight of about 17,300 and arranged in the virus with a pitch  of 23A0
The capsid has all amino acids found in other plant proteins.
Each capsomere contain about 168 amino molecules.

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