Viroids

 

Introduction

Viroids are the smallest known infectious agents, made up of only a short strand of circular, single-stranded RNA without a protein coat.

Fig: Structure of Viroids

Characteristics of Viroids:

1. Viroids are the smallest infectious agents known, discovered by Theodor O. Diener in 1971.

2. They consist only of a short strand of circular RNA (about 250–400 nucleotides).

3. They do not code for proteins and lack a protein coat (capsid).

4. Viroids mainly infect plants and cause several plant diseases (e.g., Potato spindle tuber disease).

2. Non-living and Living Characteristics

2.1 Non-living Characteristics

1. Lack capsid, metabolic machinery, and proteins.

2. Cannot survive independently outside host cells.

2.2 Living Characteristics

1. Self-replicating RNA in host cells.

2. Capable of mutation and evolution.

3. History of Viroids

1. 1971 – Discovered by Theodor O. Diener.

2. Identified as the causative agent of Potato spindle tuber disease.

3. Later linked to several crop diseases, affecting agriculture worldwide.

4. Structure

1. Composed of a small, circular, single-stranded RNA molecule.

2. No capsid, no protein coat, no lipid envelope.

3. Replicate within the host plant cell’s nucleus or chloroplast.

5. Classification / Importance

1. Pospiviroidae: Replicate in nucleus (e.g., Potato spindle tuber viroid).

2. Avsunviroidae: Replicate in chloroplast (e.g., Avocado sunblotch viroid).

3. Agricultural importance: Cause severe crop losses.

FAQs on Viroids

1. How are viroids different from viruses?
Viroids contain only RNA and no protein coat, while viruses have both nucleic acid and protein capsid.

2. Do viroids infect animals?
No, viroids mainly infect plants.

3. Why are viroids harmful to crops?
They interfere with normal plant growth, leading to stunted growth, discoloration, or crop loss.

MCQs on Viroids

1. Who discovered viroids?
(a) Stanley Prusiner (b) Theodor O. Diener ✅ (c) Wendell Stanley (d) Louis Pasteur

2. Viroids are composed of:
(a) DNA only (b) RNA only ✅ (c) Protein only (d) DNA + Protein

3. The first viroid discovered caused which disease?
(a) Mad cow disease (b) Potato spindle tuber disease ✅ (c) Influenza (d) Polio

Worksheet

Fill in the blanks:

1. Theodor O. Diener discovered __________ in 1971.

2. Viroids are infectious agents made up of __________.

3. The first viroid discovered was associated with __________ disease.

4. Viroids replicate in __________ of plant cells.