How much do you know about viroids and virusoids?

Let’s update your knowledge about the sub-viral agents – ‘Viroids and virusoids’.

What are Viroids?

Viroids are infectious agents that consist only of RNA. Viroids consist of single-stranded circular RNA that forms a seemingly double-stranded structure by intra-strand base pairing.

Table 1: Salient features of viroids

                  FEATURE                                              CHARACTER
           Infectious part It is an infectious RNA particle.
           Nucleic acid The genetic material is single-stranded RNA of low molecular weight.
           Capsid A protein coat or capsid is absent.
           Protein Viroid RNA does not code for any proteins.
           Size Viroid has a smaller size (one-fifth the size of the smallest virus).
           Host They infect only plants.

Viroids are small, covalently closed, circular, single-stranded RNA molecules which are capable of autonomous replication and induction of disease. They are the smallest known pathogens. They range in size from 250 to 370 nucleotides. These are infectious RNA molecules that differ from viruses in lacking a capsid. Despite this, a reasonably stable extracellular form travels from one host cell to another.

viroids and virusoids

Image Source: Prescott, Harley, and Kleins Microbiology 7th edition

This is the schematic diagram of a viroid which shows the general organization of a viroid. There is an enormous intrastrand base pairing as well as interspersed unpaired loops

Who discovered the viroids?

In 1971, Theodore Otto Diener, a plant pathologist at the U.S Department of Agriculture’s Research Center in Beltsville, Maryland discovered a new infectious agent. It was smaller than viruses and caused potato spindle tuber disease. Due to its lack of protein coat, it was termed ‘Viroid’. 


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Viroid diseases

A few well-studied viroids and their diseases include- 

1) Potato spindle tuber viroid (359 nucleotides) – Causes Potato spindle tuber disease.

2) Coconut cadang-cadang viroid (CCCVd) (246 nucleotides) – Causes Cadang-Cadang disease

3) Citrus exocortis viroid (371 nucleotides) – Causes Citrus Exocortis disease

What are virusoids?

Virusoids are specialized satellite RNAs that are similar to viroids as they are also covalently closed, circular, ssRNA molecules. They have specialized regions capable of intrastrand base pairing. Viroids encode one or more gene products and they particularly need a helper virus to infect host cells. This helper virus aids in the completion of its replication cycle.

Human hepatitis D virusoid is the most common virusoid. It is 1,700 nucleotides long. The helper virus for this viroid is the hepatitis B virus.

Discovery of virusoids

Virusoids were first discovered by J. W. Randles and coworkers in 1981

Hope the article could meet your needs about the sub-viral agents  ‘Viroids and Virusoids’. Stay tuned for detailed information. 

Thank you for reading at MBD and for more such information stay updated with MBD. 

Team MBD

Microbiology updates: Microbiology Archives – My Biology Dictionary

Read more on Viroids – an overview | ScienceDirect Topics

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