Model Answer
0 min readIntroduction
Viroids and viruses are both acellular infectious agents, but they represent distinct classes of pathogens. Viruses, discovered in the late 19th century, are known for their complex structures and ability to infect a wide range of hosts. Viroids, however, were first identified in 1971 by Theodor O. Diener, and represent an even simpler form of infectious particle. They lack the protein coat characteristic of viruses and consist solely of a small, circular RNA molecule. Understanding the differences between these entities is crucial for comprehending plant pathology and the evolution of infectious agents.
General Features of Viroids
Viroids are the smallest known infectious agents. They are composed solely of a small (246-401 nucleotides), circular, single-stranded RNA molecule. This RNA lacks a protein coat (capsid) and does not encode any proteins. Viroids replicate autonomously within host cells, utilizing the host’s RNA polymerase II to transcribe their RNA. They primarily infect plants, causing a variety of diseases. The mechanism of pathogenesis is not fully understood, but it is believed to involve RNA silencing pathways and interference with host gene expression.
How Viroids Differ from Viruses
The key differences between viroids and viruses can be summarized as follows:
| Feature | Viroids | Viruses |
|---|---|---|
| Genetic Material | Single-stranded circular RNA | DNA or RNA (single- or double-stranded, linear or circular) |
| Capsid | Absent | Present (protein coat) |
| Protein Encoding | None | Encode proteins (structural and functional) |
| Replication | Utilizes host RNA polymerase II | Utilizes host or viral enzymes |
| Host Range | Primarily plants | Plants, animals, bacteria, archaea |
| Size | ~246-401 nucleotides | Variable, much larger than viroids |
Mechanism of Infection & Replication
Viroids enter plant cells through wounds or via vectors like insects or contaminated tools. Once inside, they utilize the host cell’s machinery to replicate. The viroid RNA is transcribed by host RNA polymerase II, producing multiple copies of the viroid genome. These copies then accumulate within the cell, leading to disease symptoms. Viruses, on the other hand, have more complex replication cycles, often involving the synthesis of viral proteins and assembly of new viral particles.
Pathogenesis
Viroid-induced diseases are often characterized by stunting, leaf discoloration, and fruit deformation. The exact mechanisms of pathogenesis are still being investigated, but it is believed that viroids interfere with host gene expression, potentially through RNA silencing pathways. Viruses cause disease through a variety of mechanisms, including cell lysis, immune evasion, and disruption of cellular processes.
Two Diseases Caused by Viruses
Viruses cause a vast array of diseases in plants and animals. Two examples are:
- Tobacco Mosaic Virus (TMV): This virus infects tobacco and other plants, causing a characteristic mosaic pattern on the leaves. It was one of the first viruses to be discovered (1892 by Dmitri Ivanovsky).
- Influenza (Flu): Caused by influenza viruses (types A, B, and C), this is a common respiratory illness in humans. Influenza A viruses are responsible for pandemics, such as the 1918 Spanish Flu pandemic, which infected an estimated 500 million people worldwide.
Conclusion
In conclusion, viroids and viruses, while both infectious agents, differ significantly in their structure, replication strategies, and host range. Viroids represent a remarkably simple form of pathogen, consisting solely of RNA, while viruses are more complex, possessing a protein coat and encoding proteins. Understanding these differences is crucial for developing effective strategies to control plant and animal diseases caused by these entities. Further research into the mechanisms of viroid pathogenesis is needed to develop resistant plant varieties and mitigate crop losses.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.