Lecture - 36 Viruses: Characteristics and Ultra structure and Chemical nature
Viruses:- 
> These are infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria.
> The name is from a Latin word meaning “slimy liquid” or “poison.”
> The earliest indications of the biological nature of viruses came from studies in 1892 by the Russian scientist Dmitry I. Ivanovsky and in 1898 by the Dutch scientist Martinus W. Beijerinck.
Characteristics of viruses:-
1. Size:-
- On the whole viruses are much smaller than bacteria.
- Most animal and plant viruses are invisible under the light microscope.
- Some of smaller viruses are only 200Å in diameter.
2. No independent metabolism:-
- Viruses cannot multiply outside a living cell.
- No virus has been cultivated in a cell-free medium.
- Viruses do not have an independent metabolism.
- They are metabolically inactive outside the host cell because they do not possess enzyme systems and protein synthesis machinery. Thus viruses are obligatory intracellular parasites.
3. Simple structure:-
- Viruses have a very simple structure.
- They consist of a nucleic acid core surrounded by a protein coat. In this respect they differ from typical cells which are made up of proteins, carbohydrates, lipids and nucleic acids.
- Myxoviruses have a membranous envelope consisting of proteins, carbohydrate and lipid outside the usual protein coat, but this envelope is derived from the host cell.
4. Absence of cellular structure:-
- Viruses do not have any cytoplasm, and thus cytoplasmic organelles like mitochondria, Golgi complexes, ribosomes, lysosomes etc. are absent.
- They do not have any limiting cell membrane.
5. Nucleic acids:-
- Viruses usually have only one nucleic acid, either DNA or RNA.
- Typical cells have both DNA and RNA.
- Rous Sarcoma virus (RSV), producing certain cancer, is the only virus having both DNA and RNA.
6. Crystallization:- Many of the smaller viruses can be crystallized, and thus behave like chemicals.
7. No growth and division:- Viruses do not have the power of growth and division. The genetic material of virus reproduces only in a host cell.
Non-living anf Living characters of Viruses:-
a. Non-living Characters:-
i. Can be crystallised.
ii. Absence of metabolism.
iii. Inactive outside the host.
iv. Do not show functional autonomy.
v. Energy producing enzyme systems are absent.
b. Living Characters:-
i. Presence of nucleic acid and protein.
ii. Capable of undergoing mutation.
iii. Ability to proliferate or multiply inside a host organism.
iv. Ability to infect and cause diseases in living beings.
v. Showing irritability.
vi. Host-specific.
Ultra structure and Chemical nature of Viruses:-
1. Size:-
- Variable. Most viruses are much smaller than bacteria.
- The size ranges in between 100A to 250 mu.
- Some viruses are larger than bacteria, for example the psittacos is a virus measuring 0.75 mu in diameter.
2. Symmetry:-
- Viruses occur in three main shapes. They are spherical (Cubical or polyhydral), helical (Cylinderical or rod-like) and complex.
- Cubical viruses may be tetrahydral (4 faces) or dodecahedral (12 faces) or icosahedral (20 faces).
- The Herpes virus is dodecahedral.
- The Tobacco mosaic virus (TMV) and the bacteriophage are, respectively, helical and complex.
i. Spherical / Cubical:- PhI X 174, Herpes virus, Tipula virus, Polyoma virus.
ii. Helical / Cylinderical:- Tobacco Mosaic virus, Influenza virus Mumps virus.
iii. Complex:- Vaccinia virus, ORF virus, Vesicular Stomatitis virus.
3. Morphology:-
- Morphologically a virus is a core of nucleic acid (DNA or RNA) surrounded by a protein shell.
- An intact virus unit is known as virion.
- Its protein coat is called capsid.
- The capsid is composed of a number of subunits of a particular shape. These sub-units are known as capsomeres.
- The capsid protects the nucleic acid against the action of nuclease enzyme.
- Some proteins of capsid help in binding the virus to the surface of host cells.
- Some surface proteins act as enzyme and dissolve the surface layer of host cell and thus help in penetration of its nucleic acid into the host cell.
- The polio virus (Poliomyelitis) is a most extensively studied animal virus. It has a very simple organization. It consists of a protein coat built up out of 60 structurally equivalent, asymmetric protein subunits of approximately 60 Å in diameter. The spherical protein coat has a diameter about 300Å. It encloses the genetic material, RNA.
The protein coat contains about 49, 600 amino acids and RNA contains about 5200 nucleotides. The single-stranded RNA of poliovirus, thus, has triplet codes for 1700 amino acids. During infection, it alters cell metabolism drastically and leads quick death of host cell.
- Tobacco mosaic virus is the most extensively studied plant virus. It is a helically symmetrical, rod-shaped virus having the length of 3000Å and diameter of 180A. It RNA is a single stranded spirally coiled molecule formed of 6500 nucleotides. The capsid is formed of 2130 capsomeres, each with a molecular weight of 18,000. The capsomeres are elliptical and remain arranged helically around to form capsid.