How Do Virions Infect Host Cells? Step-by-Step Guide
To understand virion meaning let us say that it is a complete viral particle comprising RNA or DNA and surrounded by a protein shell that constitutes the virus's infective form. The outer protein shell of the virion is known as a capsid. Virion capsids are made from identical protein subunits called capsomeres. The RNA or DNA is situated at its inner core, confers infectivity, whereas the capsid offers specificity to the virus. In some versions of the virion, the capsid is further circumscribed by a fatty membrane wherein the virion is inactivated by exposure to fat solvents such as ethers and chloroform. The shape of many virions is spheroid, particularly an icosahedron. The capsid has 20 triangular faces with uniformly arranged unit terms as capsomeres; you can see two to five of them along each side, with the densely packed nucleic acid inside.
On the contrary, other types of virions consist of an odd number of surface spikes, and the acid is loosely coiled within. Most plants' virions are rod-shaped, wherein the capsid is a naked cylinder without a fatty membrane with a straight or helical rod of nucleic acid. The primary function of a vision is to ensure the injection of the viral nucleic acid into the host's cell. Other functions of virions include- safeguarding the genome from nucleolytic enzymes, genomic delivery, the interaction of viruses and enclosures.
Virions are known as inert carriers of genomes. They cannot grow and form through division. Small pox virus, HIV, Coronavirus, Fluvirion and Phage P-22 are certain examples of virions.
As such, we can define virion as the ineffective form of a virus outside a host cell membrane, with a nucleic acid core and a capsid.
Features of the Virion
We have already discussed the virion meaning; let us now go through some of its features.
The virion shell provides safety to the interior core, which consists of the genome and other proteins.
After binding on to the surface of a host cell, the virion’s DNA or RNA is injected onto the host cell, and viral replication occurs, leading to the spread of the infection to other host cells.
A virion is an infectious particle for transmitting the nucleic acid genome to hosts or host cells.
The cytoplasm of complex viral factories produces virions, that is, the virus.
Virions are extremely heat-sensitive, relatively cold-stable, and get inactivated by nonionic detergents, ether, and chloroform. They are comparatively resistant to photodynamic inactivation.
Virions contain at least four significant proteins of about 61, 64, 48, and 19 kDa. Scientists have also observed several minor protein bands in virion preparations.
Virions and the host cell membranes have similar fatty acid compositions.
Virions are usually 80 nanometers in diameter and have a diameter variation range of 50 to 125 nm. The size difference is due to virion heterogeneity, resulting in at least three distinct forms of virions during infection.
Structure of Virion
It is observed that virions exhibit icosahedral symmetry. Although detailed virion structure is unknown, research predicts that the capsid is structurally similar to the T=3 capsids of sobemoviruses. They are pleomorphic but typically spherical, with an average diameter of 90 to 110 nanometers. They are surrounded by a lipid envelope covered with surface glycoprotein spikes. Virions contain the L and S genome RNA s as helical NC structures organized into round configurations of lengths ranging from 400 to 1300 nanometers.
The buoyant density of a virion is Cs2SO4 is 1.32 g cm−3. Virions are stable with a pH between 6 and 8 and have an ionic strength of 0.01 to 0.1M phosphate.
Difference Between Virus and Virion
Although the terms' virus and virion may seem interchangeable, they are different and have unique variations.
A virus is a non-cellular, obligate parasite that is self-replicating inside a specific host cell. On the contrary, virion is another form of a virus.
A virus is the nucleoprotein particle, whereas virion is the active, infectious form of the virus.
A virus consists of a genetic material covered by a protein capsid, and it does not display any metabolic activity. On the other hand, Virion is the vector stage of a virus that allows the transmission of the virus from an infected host cell to another host cell.
The virus is the extracellular phase, whereas the virion is the intracellular phase of the virus.
A virus is comparatively more prominent than a vision.
The shape ranges of viruses are – helical, icosahedral, prolate, envelope, and complex. On the other hand, a virion is usually spheroidal or rod-shaped.
Similarities Between Virion and Virus
Both viruses and virions have RNA or DNA as their genetic material.
Both of them have a protein covering.
Both virion and virus are self-replicating.
Virion and virus both are non-cellular organisms that are dependent upon the host.
They exhibit meager metabolic activity.
Conclusion
Virus and virion are two forms of tiny microscopic parasitic organisms. They are both infectious, but a virus is a type of nucleoprotein, whereas a virion refers to the entire article, which is contagious. Thus, the main difference between a virus and a virion is the part they play during infection.
FAQs on What Is a Virion? Definition, Structure, and Key Functions
1. What is a virion in biology?
A virion is the complete, infectious virus particle that exists outside of a host cell. It is essentially the vehicle a virus uses to spread from one host to another. A virion consists of a nucleic acid core (either DNA or RNA) surrounded by a protective protein coat called a capsid. Some virions may also have an outer lipid envelope.
2. What is the primary difference between a virus and a virion?
The main difference lies in their state and location. A virion is the entire, assembled, and infectious virus particle as it exists outside a host cell. It is metabolically inert. The term virus is more general and refers to the pathogen at any stage of its life cycle, including when it is replicating inside a host cell and has shed its protein coat.
3. Can you describe the basic structure of a virion?
A virion has a fundamental structure designed for protecting its genetic material and infecting host cells. The core components are:
- Nucleic Acid: The genetic material, which can be either DNA or RNA, but not both. It carries the instructions for viral replication.
- Capsid: A protein shell that encloses the nucleic acid. It is made of smaller protein subunits called capsomeres and gives the virion its shape (e.g., helical, icosahedral).
- Envelope (in some viruses): An outer lipid membrane derived from the host cell, which surrounds the capsid. Viruses with this layer are called enveloped viruses, while those without are non-enveloped or naked viruses.
4. Why is a virion considered metabolically inert outside a host cell?
A virion is considered metabolically inert because it lacks the necessary cellular machinery for self-replication and metabolism. It does not have ribosomes, mitochondria, or enzymes for energy production or protein synthesis. It is essentially a dormant package of genetic information that can only become active and replicate by hijacking the machinery of a living host cell.
5. How does the structure of a virion's capsid aid in the infection process?
The capsid serves two critical functions during infection. Firstly, it protects the fragile viral nucleic acid from environmental damage, such as enzymes or pH changes. Secondly, the surface proteins on the capsid (or the viral envelope) have specific shapes that allow them to recognise and bind to receptor sites on the surface of a specific host cell, which is the crucial first step for viral entry and infection.
6. How do virions differ from other infectious agents like viroids and prions?
Virions, viroids, and prions are all non-cellular infectious agents, but they differ significantly in their composition:
- Virion: Composed of nucleic acid (DNA or RNA) and a protein coat (capsid). It is a complete virus particle.
- Viroid: Consists only of a short, circular strand of RNA. It has no protein coat and primarily infects plants.
- Prion: An infectious agent made entirely of misfolded protein. It contains no nucleic acid (DNA or RNA) and causes neurodegenerative diseases.
7. Give an example of a well-known virion.
A classic example is the Influenza virion. It is an enveloped virus containing an RNA genome. Its outer envelope is studded with proteins like hemagglutinin and neuraminidase, which are essential for attaching to and entering host cells in the respiratory tract. Another example is the virion of the bacteriophage T4, which infects E. coli bacteria and has a complex structure with a head, tail, and tail fibres.
