Grade 11 Botany Note


The branch of biology that deals with the viruses is called virology. The words virus is derived from the Latin words (Latin,venom= poisonous fluid) meaning poison.

In other words virus are known as obligatory parasites as they are fully depends on the living organism for multiplication. A mature virus particle outside the host is called virus.

History of virus

A German scientist Adolf Meyer discovered virus in 1886 A.D. He described the infectious nature Tobacco mosaic Virus (TMV). The Russian botanist Iwanowsky(1892) was first to give clear evidence of virus. Again In (1935) W.M.Stanley an American microbiologist for the first time was able to crystallize the Virus causing Tobacco Mosaic Diseases (TMV). After Isolating from Infected tobacco leaf juice. He thus showed that viruses are not like typical cell.

Stanley was awarded Noble prize for his work.  Barden (1943A.D.) define the virus “as an obligatory parasitic pathogen having diameter of less than 200µm”.

Characteristics of Viruses

1. All the viruses are minute and simplest organism. The can only be seen with an electron microscope.

2. The diameter of viruses ranges from about 20-30µm.

3. They show respond to chemicals, heat and temperature.

4. They are highly specific to their host.

5. They undergo mutation like living organism.

6. Virus occurs in mainly 3 main shapes respectively;

Spherical, helical, complex.


Nature of viruses

They exhibit the properties of both living and nonliving things.



Living characteristic of viruses

 1. Viruses can grow and reproduce within the host cells.

2. They undergo mutation like other organism.

3. They have their own DNA or RNA or both.

4. They are capable of transmitting disease.

5. They are sensitive to light and temperature.


Nonliving characteristic of Viruses

1. They lack of cytoplasm, cell organelles and may be nucleus.

2. They do not respire.

3. They are heterotrophic, i.e. they depend upon other for their living.

4. In viruses respiration process not occurs.

Classification of viruses

They are classified on the basic of type of host and genetic material.

On the basic of type of host they are as,

1. Animal viruses:  the virus which live inside the animal cell. E.g. influenza virus, rabies virus, polio virus etc. their genetic material is RNA or DNA.

2. Plant viruses:  The viruses which live inside the tobacco mosaic virus (TMV), potato virus (TV), beet yellow virus (BYT). Their genetic material is RNA.

3. Bacteriophages: the viruses which infect bacterial cells are known as bacteriophages.

On the basic of nucleic acids they are as

1. DNA virus: they possess DNA is a genetic material.

2. RNA virus: they possess RNA as a genetic material.

3. DNA RNA viruses: they possess RNA DNA as a genetic material.

Structure of bacteriophages

 The shape and size of bacteriophage different from species to species. Some are spherical and some of them are comma shaped while majority of them have tadpole- like structure. T2 T 4T 6 Phases are tadpole- shaped with polyhedral head, short neck, collar and straight tail. The shape of head is bi-pyramidal hexagonal in shape. Head is enclosed by a capsid which is about 15oA thick. The head encloses the double stranded greatly folded DNA contain more than 75 genes. Where the tail is hollow in structure rest on a hexagonal end plate that bears six tails.                         

Bacteriophage Structure

Fig: structure of t2 phase.



Economic importance of Virus

1. Positive effects

- They are use in laboratory for various researches.

- They can also destroy some bacterial cells in plant.

-The virus such as bacteriophages can also use in the treatment of several bacterial disease like cholera.

- Virus also helps to maintain ecosystem.

2. Negative effects

-Due to virus many plant species can be affected.

-virus also destroys the crop production, e.g. TMV.

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