What is the Structure of a Virus? | History and Characteristics

Definition of virus

The virus word is originated from the Latin word “venom” which means "poisonous". A virus is a non-cellular infectious agent containing DNA or RNA in a protein coat and cannot replicate without a living host. A virus is smaller than a bacterium in size and cannot be seen through a simple microscope.

There is a whole branch of biology that deals with the structure, function, replication, and diseases of viruses called virology.

History of virus discovery

In the 19th century, at the time of Louis Pasteur, viruses were normally referred to as toxic substance that causes disease and death.

In 1884, Charles Chamberland, a colleague of Louis Pasteur, discovered that causative agents of rabies disease can pass through porcelain filters, however, bacteria cannot pass through that filter. So, they described that filterable disease-causing toxic substances as filterable viruses.

In 1892, Ivanowski gained bacteria-free filtrate of plants suffering from tobacco mosaic disease and discovered that this disease-causing agent was filterable.

Similarly, causative agents of foot and mouth disease (1898) and yellow fever (1901) were discovered in the filtrate.

Stanley was the first scientist to successfully crystalized filterable agents of the tobacco mosaic virus in 1935. The chemical analysis of these crystals showed the presence of nucleic acid and protein.

Video lesson

What is the structure of a virus?

A completer virus having the ability to cause infectious and present outside the host cell is called a virion. 

Structure of a virus.

The structure of a virion is made up of the following parts:

Genome

The central part of a virus contains nucleic acid either DNA or RNA which is known as the genome. The genome contains information about the viral protein.

The Capsid

The genome is surrounded by a protein coat called the capsid. Capsid provides a definite shape to the virus. The subunits of proteins called capsomeres combine to form capsid. Each virus has a specific number of capsomeres in the capsid. For example, the herpes virus contains 162 capsomeres, while the adenovirus has 252 capsomeres in its capsid.

The Envelope

Some animal viruses have another covering outside the nucleic acid and capsid (nucleocapsid) which is derived from the host cell membrane called the envelope. It protects the genetic material of the virus while moving from one host to another during their life-cycle. Viruses having no envelope are called naked viruses.

Envelope proteins

On the surface of the envelope, viral glycoproteins are present called envelope protein. These proteins help in the identification and binding to the receptor site of the host cell membrane. They are also helpful in protecting viruses from the host immune system.

Enzymes

RNA viruses contain only RNA. So, RNA viruses encode enzymes that can replicate RNA into DNA to replicate their genomes in the host cell. Such RNA polymerase enzymes make more copying errors during transcription than DNA polymerases. This is the reason for the high mutation rate in RNA viruses compared to DNA viruses.

What are the characteristics of viruses?

Viruses are different from other living organisms and have some interesting characteristics such as:

• They are very small in size ranging from 250 nm (poxviruses) to 20 nm (parvovirus).
• They cannot be seen under a simple microscope.
• They are 10 – 100 times smaller compared to bacteria.
• They require a living host for their growth and cannot be grown on artificial media.
• They can pass easily through the pores of a filter.
• Viruses lack metabolic machinery to synthesis their protein and nucleic acids.
• They may cause disease in the host during their replication.
• They are resistant to antibiotics such as streptomycin and penicillin.
• Viruses are found in various shapes. They may be helical, polyhedron, and enveloped.

Some Answers and Question

1. Who first discovered the virus?
A. In 1887, Dmitry Ivanovsky first discovered the virus of Tobacco Mosaic Disease.

2. When was the first human virus discovered?
The virus causing yellow fever was the first human virus to be discovered in 1901.

3. Which is the smallest animal virus?
The smallest animal virus is parvovirus having a size of about 20 nm.

4. What is capsid in a virus?
A. The genome of the virus is surrounded by a protein coat called the capsid.

5. Is a virus a living organism?
A. No, the virus is not considered as living because it cannot reproduce by itself and needs host machinery for replication.

6. From where does the virus get its envelope?
A. Some animal viruses have another covering outside the nucleic acid and capsid (nucleocapsid) which is derived from the host cell membrane called the envelope.

7. Define naked virus?
A. Any virus having no envelope around its nucleocapsid is called a naked virus.

8. Do all viruses contain DNA as genetic material?
A. Not all viruses contain DNA as genetic material, some viruses have RNA. Some viruses contain single-stranded while some have double-stranded DNA present in their capsid.

9. State the function of envelope proteins in the virus.
A. Envelope proteins help in the identification and binding of the virus to the receptor site of the host cell membrane. In their absence, the virus cannot identify the receptor site and binding will not happen.

10. What are the different shapes of viruses?
A. Viruses are found in various shapes. However, the three main shapes are helical, polyhedron, and enveloped.

References

• Bamford, D. H., Grimes, J. M., & Stuart, D. I. (2005). What does structure tell us about virus evolution?. Current opinion in structural biology, 15(6), 655-663.
• Harrison, S. C. (2001). Principles of virus structure. Fields virology, 1, 53-85.
• Zlotnick, A. (2005). Theoretical aspects of virus capsid assembly. Journal of Molecular Recognition: An Interdisciplinary Journal, 18(6), 479-490.