The document summarizes key information about viruses, including their discovery, structure, and characteristics. It provides details on different virus shapes (helical, polyhedral, complex) and structures. Specific viruses discussed include tobacco mosaic virus (TMV), adenovirus, bacteriophage T4, and their components. Viruses are described as ultramicroscopic particles that can only be seen with electron microscopes and vary greatly in size and shape. They contain nucleic acids and infect host cells to reproduce.

1. R. NITHYA M. Sc., M. Phil., PGDCA., (Ph. D)
ASSISTANT PROFESSOR IN BIOTECHNOLOGY
SACWC, CUMBUM,THENI DT,
TAMIL NADU, INDIA.

2. VIRUS
DISCOVERY
The existence of virus was first proved by Ivanowski in 1892.
The first virus was discovered by Ivanowski in 1899.
Ivanowski

3. GENERAL PROPERTIES OF VIRUS
 Viruses are sub-microscopic, self reproducing particles capable of being introduced
into living cells and reproducing such cells only.
 Virus means poison in Latin.
 They are intermediate between living and non-living things.
 They are neither prokaryotes nor eukaryotes.
 They are simplest form of life. They are parasitic.
 They are called living chemicals.
 They range in size from 100A0 to 2,500A0. The shape of the viruses varies.
 They may be rod shaped, bullet shaped, brick shaped, spherical, oval or irregular.

4. CHARACTERISTICS OF VIRUSES
Viruses are extremely smaller in size.
They are potentially infectious.
They have a single stranded nucleic acid either DNA or RNA (except RNA-DNA viruses).
Viruses do not contain cellular structures such as, plasma membrane, mitochondria,
golgi complex, lysosomes, ribosomes, etc.
They do not contain information for the production of enzymes in energy cycle.
Viruses are intracellular parasites.
They live inside cells of plants, animals and bacteria.

5. STRUCTURE OF VIRUSES
Viruses are ultramicroscopic.
They can be seen only by an electron microscope.
Viruses are smaller than bacteria.
The smallest virus is 10 nm in diameter E. g. Parvo virus.
The largest virus is about 250 nm.
The shape of viruses varies.
They may be rod shaped (E. g Pox viruses), bullet shaped,
brick shaped, spherical, oval or irregular.
Pox viruses

6. SHAPE OF VIRUS
Viruses occur in three main
shapes, namely
Helical
Polyhederal
Complex

7. HELICAL VIRUSES
In helical viruses, Capsid and nucleic acid are helically coiled
The capsid is made up of 2,130 monomeres.
The monomeres are arranged in 130 turns
E. g. Tobacco mosaic virus
Influenza virus
Mumps virus etc.

8. Tobacco mosaic virus
Plant virus, it is rod shaped RNA virus.
It causes diseases called tobacco mosaic in tobacco plants
It is about 3000 A long and 170 A diameter.
It is made up of two components
Outer capsid
inner RNA
Capsid
Outer coat
Formed of protein
Capsid is made up of small capsomeres
There are about 2,130 capsomeres in a TMV
Capsomeres are helically arranged around the RNA.
There are about 129 complete spirals
RNA
It is single stranded nucleic acid
There are about 3 nucleosides in the RNA per capsomere
There are about 6,400 nucleotides in the RNA

9. POLYHEDERAL
In polyhedral viruses, the capsid are many sided.
They are of three types, namely
Tetrahedral
Octahedral
Icosahedral
Tetrahedral - In tetrahedral viruses, the capsid has 4 sides
Octahedral – In octahedral viruses, the capsid has 8 sides.

10. Icosahedral –
The capsid has 20 triangular facets and 12 corners.
Most of the viruses are icosahedral.
The icosahedral capsid is made up of many
subunits called capsomeres.
There are two types of capsomeres, namely
pentameres and hexameres.
The pentamere (Pentons) is made up of 5 monomeres.
Hexameres – 6 monomeres
Capsomere
Made up of poypeptide chains
Number of capsomeres will be either
12,32, 4, 72, 92, 162, 252, 362, 492, 642 or 812.

11. ICOSAHEDRAL CAPSID

12. Pentons are 12 in number present at the vertices or corners
It contain a fibre and a knob
Hexons varies in number present on facets or sides.
Icosahedral capsid are found in viruses such as
PHIx174
Adeno virus
Turnip yellow mosaic virus
Herpes virus
FUNCTIONS OF CAPSID
Gives shape to virus
Protects nucleic acid
Helps in attachment of the virus to host cell.
Adeno virus

13. COMPLEX VIRUS
In complex viruses, the capsid may be absent or the capsid may
be attached with additional structures.
E.g. Vaccinia virus, phages of T-even series.
The T2 bacteriophage, shows a combination of
icosahedral symmetry
and helical symmetry

14. STRUCTURE OF BACTERIOPHAGE
It is a bacterial virus
Living inside bacterial cells
Bacteria eating virus
First described by Twort in 1915.
Common bacteriophage is T4 bacteriophage
Parasitic on human colon bacteria E. coli.
So it is called coli phage.
T4 phage is tadpole shaped
Consists three parts
A head,
Neck and
tail

15. HEAD
Polyhedral in shape
Covered by a protein coat called capsid.
Capsid is made up of about 2000 protein subunits called
capsomeres
Double stranded DNA is present inside the head
DNA is highly coiled and tightly packed contain more than 75
genes.
NECK
It contains collar
It connects the head with tail
TAIL
Consist a central hollow core tube
The tube is covered by a sheath or tube made up of about 144
protein Contractile subunits
Free end of core tube has a hexagonal end plate
The end plate has 6 spikes and 6 fibres. Spikes are used for
penetration. Fibres are used for attachment on the host