Difference Between Virus and Virion

Main Difference – Virus vs Virion

Virus is a non-cellular, obligate parasite that is self-replicative inside a specific host cell. Virion is another form of a virus. The main difference between virus and virion is that virus is the nucleoprotein particle whereas virion is the active, infectious form of the virus. A virus consists of genetic material covered by a protein capsid. It does not show any metabolic activity. Virion is the vector stage of a virus, which allows the transmission of a virus from an infected, host cell to another host cell. Therefore, the virus is the extracellular phase whereas virion is the intracellular phase of the virus.

Key Areas Covered

1. What is a Virus
     – Definition, Structure, Role
2. What is a Virion
     – Definition, Structure, Role
3. What are the Similarities Between Virus and Virion
     – Outline of Common Features
4. What is the Difference Between Virus and Virion
     – Comparison of Key Differences

Key Terms: Capsid, DNA, Obligate Parasites, RNA, Virion, Virus

What is a Virus

Virus refers to a small parasite that consists of a nucleic acid molecule covered by a protein coat. It consists of a non-cellular organization and specific modes of reproduction. A virus is essentially a non-living entity as it does not show any metabolic activity. Typically, a virus particle consists of either a single-stranded or double-stranded, DNA or RNA genome, covered by a protein coat known as the capsid. Viruses have various sizes that range from 10–400 nm in diameter. The different shapes of a virus are helical, icosahedral, prolate, envelope, and complex. The smallest viruses such as picornavirus are little larger than ribosomes. The largest viruses such as poxviruses can be viewed under a light microscope. However, scanning or transmission electron microscope should be used for the observation of most viruses. The structure of an icosahedral virus is shown in figure 1.

Figure 1: Icosahedral Virus

Viruses can only reproduce inside a specific host. Generally, viruses use host cell mechanism for the synthesis of new virus particles. The two types of viral reproduction methods are the lytic cycle and lysogenic cycle. During the lysogenic cycle, the synthesis of viral particles does not cause the host cell lysis. However, the host cell is lysed during the replication of viral particles. Some examples of viruses and the diseases they cause in humans are described in the table below.

Examples of Viruses and Diseases

What is a Virion

Virion refers to the complete, infective form of a virus outside the host cell. A virion is a complete virus particle, composed of either a DNA or RNA genome, and covered by a protein capsid. The genetic material is responsible for the infectivity while the proteins in the capsid are responsible for the specificity of the virus. Generally, a viron is the small, extracellular form of a virus. Virion consists of an additional outer membrane made up of lipids known as the envelope. These type of viruses can be inactivated by the exposure of the virus to organic solvents such as ether or chloroform. The structure of the IHV virion is shown in figure 2.

Figure 2: HIV Virion

Most virions are spheroidal in shape, consisting of an icosahedral viral capsid with 20 triangular faces with regularly arranged units called capsomeres. The genome is densely coiled within the capsid. Some virions consist of a capsid with an irregular number of surface spikes. Virions of most plant viruses are rod-shaped. Their capsid resembles a naked cylinder.

Similarities Between Virus and Virion

• Both virus and viron contain DNA or RNA as genetic material.
• Both virus and virion are covered with a protein coat.
• Both virus and virion are self-replicative particles.
• Both virus and virion are non-cellular organisms that are host specific.
• Both virus and virion exhibit less metabolic activity.

Difference Between Virus and Virion

Definition

Virus: Virus is a small parasite that consists of a nucleic acid molecule covered by a protein coat.

Virion: Virion is the complete, infective form of a virus outside the host cell.

Significance

Virus: Virus is the nucleoprotein particle.

Virion: Virus is the infectious form of a virus.

Intracellular/Extracellular

Virus: Virus is the intracellular structure.

Virion: Virion is the extracellular structure.

Size

Virus: Virus is comparatively large.

Virion: Virion is smaller than the virus.

Shape

Virus: The shapes of a virus are helical, icosahedral, prolate, envelope, and complex.

Virion: Most virions are spheroidal or rod-shaped.

Conclusion

Virus and virion are two forms of small obligatory parasites. Both virus and virion are infectious. But generally, virus refers to a type of nucleoprotein and virion refers to the complete particle that is infectious. Therefore, the main difference between virus and virion is the role they play during infection.

Reference:

1. Lodish, Harvey. “Viruses: Structure, Function, and Uses.” Molecular Cell Biology. 4th edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.
2. “Virion.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 27 Mar. 2017, Available here.

Image Courtesy:

 1. “CowpeaMosaicVirus3D” By Thomas Splettstoesser (www.scistyle.com) – Own work (rendered with Cinema 4D), (CC BY-SA 3.0) via Commons Wikimedia 
2. “HIV Virion Structure” by NIAID (CC BY 2.0) via Flickr