Differences between Viroid and Virion

Prepared and Presented byDeepika Rana

1601M.Sc. Microbiology

1st year

What is a Virion?•A virion is a complete functional virus that has the capacity to infect living tissue. This means that it includes the genetic material, the capsid, the envelope and the membrane proteins that allow the virus to bind to its host and enter it. •Structural unit of the virus•It has two essential structures: DNA or RNA and capsid.•Sometimes, glycoprotein spicules can be added to these basic structures•VIRUSES can be found either inside a cell (intracellular) or outside of a cell (extracellular). If it is found extracellular, the virus is called a virion. A virion contains a protein coating called a capsid, which surrounds the core of the virus containing the nucleic acid (either DNA or RNA).

•Bio-molecules found in virions: genetic material, either DNA or RNA, single or double stranded, nucleoprotein capsid, maybe an envelope usually receptor proteins or enzymes that permit binding or entry into the host.•It is an extracellular virus consisting of a protein coat (capsid) surrounding a nucleic acid core, either DNA or RNA, together known as a nucleocapsid called an envelope.•A virus will not have an envelope within a cell. If the cell was burst artificially, then these virus particles cannot be called virion because they will lack certain proteins that will make them infectious even though the genetic material is present. •Although viruses lack cell membrane, some viruses have a viral membrane surrounding its capsid. These are called enveloped virion. One without an envelop is called a naked or non-enveloped virion.

• The virion shell or capsid protects the interior core that includes the genome and other proteins. After the virion binds to the surface of a specific host cell, its DNA or RNA is injected into the host cell and viral replication occurs, resulting in the spread of the infection to other host cells.

• A virion is the infectious particle that is designed for transmitting the nucleic acid genome among hosts or host cells.Virions are produced in the cytoplasm of complex viral ‘factories,' the virus.

An HIV Virion

What is Viroid?•Viroids are infectious, non-protein‐coding, highly structured small circular ribonucleic acids (RNAs) able to replicate autonomously and induce diseases in higher plants. •Infectious agents composed only of closed, circular ssRNAs•Do not encode gene products•Requires host cell DNA-dependent RNA polymerase to replicate•Cause plant diseasesSome found in infected host cell nucleolus, others found in chloroplastMay cause disease by triggering RNA silencing

•Viroids and viruses differ in structure, function and evolutionary origin •Viroids move intracellularly, cell to cell through plasmodesmata, ‐ ‐and long distance through the phloem.•Viroids may infect their host plants latently or induce different pathogenic alterations including death.•Viroids, the smallest known pathogens, are naked, circular, single-stranded RNA molecules that do not encode protein yet replicate autonomously when introduced into host plants.•Symptoms of viroid infection in plants include stunting of growth, deformation of leaves and fruit, stem necrosis, and death.•e.g., Potato spindle Tuber Viroid Coconut Cadang Cadang Viroid

VIRION

VIROID

It is a nucleoprotein particle. It is an RNA particle.

Nucleic acid can be DNA or RNA. Viroid is formed only of RNA.

A protein covering of coat is present. A protein coat is absent.

They have a larger size. They have a smaller size.

Virions infect all types of organisms. Viroid infects only plants.

VIRION VIROID

Width- 10-300 nm Width- 2 nm

Length- 20-800 nm Length- 40-130 nm

Cytoplasmic membrane is absent but sometimes membranous envelope may be present.

Cytoplasmic membrane absent.

Responsiveness: some bacteriophages respond to a host cell by injecting their genomes.

No responsiveness.

It cannot replicate on its own. It utilizes the replicating machinery of the host cell for its replication.

It replicates by rolling circle mechanism using an RNA template. The enzyme involved is RNA polymerase II.

References 1. Prescott’s Microbiology 7th edition Wiley et al2. Agrios Plant Pathology 5th edition3. www.virology.ws4. www.els.net 5. mcmanuslab.ucsf.edu 6. Britannica 7. www.majordifferences.com 8. http://virology-microbiology-b.blogspot.in/2009/01/prions-viriods-and-virusoids.htm

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9. Google images10. Wikipedia

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