Viruses and Viroids

Introduction

Viruses are tiny, non-cellular infectious organisms that can only multiply within a host cell. Viruses cannot be classed as either living or non-living organisms from a biological standpoint. However, they have some distinctive characteristics of living and non-living elements. Viroids are smaller than viruses and have circular strings of ribonucleic acids (RNA) with no protein covering. Viroids are known to only infect plants.

Viruses and Viroids

Virus

A virus is a non-cellular, infectious organism composed of genetic material and protein. It can penetrate and replicate within the live cells of bacteria, plants and animals. A virus cannot reproduce outside of the host cell as viruses lack the required cellular machinery. As a result, it penetrates and binds itself to a particular host cell, injecting its genetic material. Further, the virus multiplies using the host genetic material and eventually rips open, discharging the new viruses.

Viruses may also crystallise, something that no other living cell can achieve. Viruses are microscopic, varying in length from 30 to 50 nm. They often lack a cell wall but are encased in a shielded protein covering- the capsid. It is distinguished by the simultaneous evolution of the virus and the host. It may be viewed as a genetic element. 

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Characteristics of Viruses

• They are non-cellular creatures that are surrounded by a protective envelope
• The existence of spikes aids in the attachment of viruses to host cells
• These viruses do not grow, breathe or metabolise, but they replicate
• They have a protein shell – capsid and a nucleic acid core composed of DNA or RNA
• They are classified as both living and nonliving entities. When these viruses are located outside of host cells, they are dormant, but they turn active when they enter host cells. These viruses induce various diseases and replicate within the host cell by utilising enzymes and raw materials

Classification of Viruses

• Based on nucleic acid present in the virus
• DNA Virus
• RNA virus

Structure or symmetry

• Complex virus
• Radial symmetry virus
• Cubical or icosahedral symmetry shaped virus
• Rod or Spiral shaped or helical symmetry virus

Replication Property

• Replication within the cytoplasm of the host cell
• Replication within the nucleus and the cytoplasm of the host cell
• Replication within the nucleus of the host cell
• Replication of the virus through the double-stranded DNA intermediate
• Replication of the virus through a single-stranded RNA intermediate

Classification based on the host range

• Animal viruses
• Plant viruses
• Bacteriophage
• Insect virus

Mode of transmission

• Airborne
• Faecal and oral
• Sexually-transmitted diseases
• Transfusion-transmitted infections
• Zoonoses

Viroids

Viroids are infectious pathogens that solely damage plants. Thus, they are also known as plant pathogens. These creatures use plant cells’ biological machinery to generate new copies of themselves. It generally affects all higher plant types.

Viroids are structurally and morphologically distinct from viruses. These are only short circular and single-stranded RNA strands, with no protein coverings. Plants infected with viroids are accountable for crop failures and a loss of vast amounts of money in agricultural earnings each year. Potatoes, tomatoes, cucumbers, flower plants and coconut palms are among the plants impacted by these viruses.

T.O. Diener found viroids for the first time in 1971. Scientists initially studied it in the potato spindle tuber viroid, which caused a massive loss to the potato industry. Viroids are plant parasites that function similarly to the transcriptional machinery of cell organelles such as the nucleus or the chloroplast because they have non-coding regions. They mainly attack the hosts’ epidermis after inflicting surface defects to the plant’s cell wall. The RNA–RNA transcription pathway replicates these.

Characteristics of Viroids

Some of the characteristics of viroids are as follows:

• Viroids are composed entirely of RNA
• These are thought to be smaller in size and exclusively infect plants
• These are some of the tiniest known infectious disease agents
• Viroids are a kind of nucleic acid with less molecular weight and a distinct form
• They replicate within the host cell, which they damage and create changes, resulting in death
• Viroids are divided into Pospiviroidae (nuclear viroids) and Avsunviroidae (chloroplastic viroids)
• Viroids are supposed to migrate intracellularly, cell to cell via the plasmodesmata, and long-distance via the phloem

Citrus exocortis, cucumber pale fruit and chrysanthemum stunt are diseases induced by viroids in plants. The proliferation of seeds in plants transfers these infectious illnesses through cutting, tubers and other means and the misuse of contaminated equipment.

The signs of viroid infection in plants include stunted growth, stem necrosis, distortion of the leaves and fruits, and eventually death. The bulk of the viroids is thought to infect plants, notably coconut and apple trees. The potato spindle tuber viroid (PSTVd) leads to considerable crop damage to potato yields by forcing tubers to stretch and eventually break. Stunting and leaf epinasty are two more frequent viroid infection signs.

Difference between Virion and Viroid

Viruses and viroids are two types of infectious particles that can only replicate within a host cell. The primary variation between viruses and viroids is that viruses are tiny infectious agents that can only multiply within live cells. In contrast, viroids are the tiniest infectious agents that attack plants. Additionally, the virus is a nucleoprotein particle with either DNA or RNA nucleic acid, whereas viroids are RNA particles. Furthermore, viruses have a protein covering known as the capsid, whereas viroids do not. 

Conclusion

Viruses are microscopic, non-cellular infectious organisms that can only replicate within the confines of a host cell. From a biological viewpoint, viruses cannot be classified as either living or non-living creatures. However, they possess certain features of both living organisms and non-living substances. Viroids are smaller than viruses and feature circular ribonucleic acid strings with no protein coating. Viroids are known to infect only plants.