Viruses are the smallest and simplest life form, composed of either DNA or RNA surrounded by a protein coat. They are non-cellular and can only replicate inside of a host cell. The first virus was discovered in 1892 in tobacco plants. Viruses come in different shapes (helical, polyhedral, enveloped) and sizes from 5-300 nm. They infect hosts through direct contact, indirect contact, common vehicles like food/water, or through airborne transmission. Viruses have a nucleocapsid core made of nucleic acids and proteins that make up the structure.

1. B.Sc.I
Diversity of Microbes And Cryptogams
Unit- I
General Account of Virus
DR. SWATI V. PUNDKAR
Assistant Professor
Department of Botany
Shri Shivaji Science College, Amravati
NAAC Accredited ‘A’ Grade

2. Definition:
Viruses are ultra-microscopic, non-cellular living particles, composed solely of a nucleic acid
(DNA or RNA) core, surrounded by a protein envelope called capsid.

3. Viruses are the most primitive acellular and no – cytoplasmic
infectious agents.
The Russian botanist D.J.Iwanowski (1892) first discovered virus
in an infected tobacco plant.
M. W. Beijerinck (1898) coined the term Viruses.
Then American chemist W. M. Stanley (1935) isolated pure
crystal of Tabacco Mosaic Virus (TMV) and conclude that virus
are made of nucleoprotein.

4. Are viruses alive?

5. Virus structure

6. How do viruses infect?

7.  Direct contact transmission: This refers to transmission via
physical contact between an infected and uninfected subject
through kissing, biting, or sexual intercourse, for example.
 Indirect transmission: Here, the virus is transmitted via contact
with contaminated objects or materials such as medical equipment
or shared eating utensils.
 Common vehicle transmission: This transmission mode refers to
when individuals pick up the virus from food and water supplies
that are contaminated with feces. This often causes epidemic
disease.
 Airborne transmission refers to the respiratory infection that
occurs when the virus is inhaled.

9. Sizes and Shapes of Viruses
Viruses are usually much smaller than bacteria with the vast majority
being submicroscopic . While most viruses range in size from 5 to 300
nanometers (nm)

10. Shapes
a)Helical viruses consist of nucleic acid surrounded by a hollow protein
cylinder or capsid and possessing a helical structure

11. b)Polyhedral viruses consist of nucleic acid surrounded by a
polyhedral (many-sided) shell or capsid, usually in the form of an
icosahedron
 Transmission electron micrograph of Adenoviruses.
Transmission electron micrograph of poliomyelitis viruses

12. c)Enveloped viruses consist of nucleic acid surrounded by
either a helical or polyhedral core and covered by an envelope
 Transmission electron micrograph of Hepatitis B viruses
 Transmission electron micrograph of an Influenza A virus
 Transmission electron micrograph of HIV

13. d)Binal (complex) viruses have neither helical nor polyhedral
forms, are pleomorphic or irregular shaped , or have complex
structures.
 Transmission electron micrograph of the
bacteriophage coliphage T4.

15. Chemical Structure
Viruses have very simple structure. The core of the virus is called a
nucleocapsid made up of nucleic acid, which is surrounded by a protein coat
called Capsid.
The nucleic acid always contains only a single kind of nucleic acid i.e. either
DNA or RNA.
A) Nucleic Acid –
B) Proteins-
1-Strutural
2- Internal Core
3- Envelop
4- Viral enzymes

16. C) Virus carbohydrates-
D) Viral Lipid
Types of Viruses
1) Plant viruses
2)Animal viruses
3)Bacteriophages
Bacteriophages