What are Viroids? Overview and Examples

Brittany W., Artem Cheprasov
  • Author
    Brittany W.

    Brittany has a bachelor's degree in biological sciences and biochemistry.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

Understand what viroids are. Understand how prions replicate and know if prions have nucleic acid. Learn the difference between viruses, viroids, and prions. Updated: 02/02/2022

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What Are Viruses?

Viruses are infectious agents that consist of genetic material wrapped in a protein coat. Viruses can only replicate (copy and grow in number) when they are inside the host cell of an infected organism. This is because they need the cellular machinery of the host to replicate successfully and survive. Viruses can cause significant damage to the host that often results in death.

Viruses are made of either single-stranded or double-stranded genetic material. There are two types of genetic macromolecules (large molecules that contain a high number of atoms):

  • Ribonucleic acid (RNA) is most often single-stranded
  • Deoxyribonucleic acid (DNA) is most often double-stranded

Most viruses contain a protein coat called a capsid. Capsids are made of various oligomeric (containing more than one subunit) proteins that enclose the genetic material.


A virus contains genetic information enclosed in a protein coat.

An illustration of the anatomy of a virus.


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  • 0:00 Non-Living Killers
  • 0:36 What Are Viruses?
  • 2:15 What Are Viroids
  • 3:03 What Are Prions?
  • 4:09 Lesson Summary
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What Are Viroids?

What are viroids? Viroids are single-stranded circular RNA molecules without a protein coat. They are relatively small infectious agents that mostly use plants as their host. Therefore, they often cause plant diseases and present significant challenges to the agricultural community. Viroids are likely passed from one plant to another via propagation techniques and other horticultural practices that involve contaminated tools. Because viroids do not have surrounding proteins like a capsid, they are only able to infect plants that have prior damage. Any damage or open wound on a plant gives them access to infect certain cells.

Even though viroids contain single-stranded RNA, specific nucleic acids (the building blocks of RNA and DNA) on one side of the circular strand can form bonds with nucleic acids on the other side of the same strand. This allows viroids to form a rod-like structure.

Viroid Examples

Viroids mostly affect plants. Common viroid examples include:

  • Potato spindle tuber viroid
  • Tomato planta macho viroid
  • Avocado sunblotch viroid
  • Peach latent mosaic viroid

Potato spindle tuber viroids cause:

  • Slower sprouting
  • Plant deformities

Tomato planta macho viroids cause:

  • Chlorophyll loss
  • Brittle leaves
  • Deformed leaves
  • Small crop yield

Avocado sunblotch viroids cause:

  • Low crop yields
  • Poor quality avocados

Peach latent mosaic viroids cause:

  • Necrosis (cellular death)
  • Wounds that lead to fungal and bacterial infections

Viroids lead to low crop yields and have the ability to affect agricultural production on a large scale. This can lead to food scarcity and socioeconomic disparities in food.

What Are Prions?

Prions are infectious agents made up of entirely misfolded protein. When protein is synthesized in the cell, one of the last steps is for it to be folded, so that it can maintain its intended function. A misfolded protein can cause damage to the cell, or even lead to disease. Sickle cell anemia is a genetic disorder that results from the misfolding of the blood protein hemoglobin.

However, prions microbiology is rare because prions are misfolded proteins without genetic material that are also transmissible (able to be passed from one organism to the other) and infectious (transmitted through the environment with the ability to cause disease). Prions are most known for causing a neurodegenerative disease called transmissible spongiform encephalopathy (TSE) where the prions accumulate in the brain, form plaques, and cause disruptions to the central nervous system. TSE is related to a family of diseases that includes:

  • Kuru
  • Fatal familial insomnia
  • Gerstmann-Straussler-Scheinker disease
  • Creutzfeldt-Jakob disease (CJD)
  • Bovine spongiform encephalopathy (BSE or Mad cow disease) in cattle
  • Scrapie in sheep and goats
  • Chronic wasting disease in elk and deer

Symptoms include:

  • Sponge-like brain tissue
  • Brain damage and dysfunctional cognitive ability
  • Loss of motor (movement and speech) function
  • Dementia

CJD in humans is caused by eating beef from cattle with BSE. Currently, there is no cure for TSE-related diseases, and most are often fatal.

How Do Prions Replicate?

The replication of prions is quite fascinating. When a prion enters a healthy organism, it incites existing, healthy, and properly-folded proteins to take on a misfolded shape. The newly misfolded proteins can then incite more healthy proteins to do the same. In this way, prion growth is rapid and exponential.

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Frequently Asked Questions

What are viroids and prions?

Viroids and prions are non-living infectious agents. Viroids are made of nucleic acids (RNA), and prions are made of amino acids (protein).

Can viroids infect humans?

Yes. However, they usually only infect plants. Viruses and prions are both non-living infectious agents that have the ability to infect humans.

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