Introduction

There are many microorganisms present around us. Some of them are beneficial, while some are harmful. We all have suffered from symptoms of the common cold that include runny nose, sneezing and feeling feverish at some point in our life. The doctor does not prescribe an antibiotic, an effective medicine against bacterial infections, for the common cold as common cold occurs due to an infection of rhinovirus. Viruses are a special class of infectious agents. They infect all types of organisms, including Archaea, Bacteria and Eukarya (Animals and Plants). There are many encyclopedias of plant viruses and viroids present, claiming to provide details of all the microorganisms. However, it is nearly impossible to provide information about all the microorganisms in a single text. 

Louis Pasteur, who is known as the father of modern microbiology, invented the Chamberland-Pasteur filter. This invention leads to the discovery of viruses and the establishment of virology.

Viruses and Viroids

Viruses: Viruses can be defined as small obligate, intracellular particles seen only by electron microscope that infects and takes over the host cell to replicate. All viruses are acellular entities and are made of genetic material, DNA or RNA, inside a protein coat. However, they do not have organelles, cell walls, cytoplasm, nucleus, or nucleoid. Their size ranges from 20-400 nm in diameter, and thus, they are not even visible under the light microscope.

Viruses exist in two phases-

1. They do not have the ability to grow and multiply in any artificial media, as they are metabolically inert. It is usually called a virion.
2. They are metabolically active when present inside a living host cell, as the enzymes required for the process are present in the host. Using them, they are able to synthesize proteins, nucleic acid, and ATP. They can multiply in this phase as the machinery required for the process is present in the host cells. They replicate using the host’s genome.

The living and nonliving characteristics of the viruses

Viruses are considered living as

1. They are made of genetic material, such as DNA or RNA and protein covering.
2. They are able to replicate only inside the host cell. They use the cellular machinery of the cell (tRNA, ribosomes and enzymes) to replicate its genome and synthesise protein coats.
3. Even the crystallized virus conserves the living properties.
4. They are transmitted from the infected host to the uninfected host.

Viruses are considered non-living as

• Viruses are unable to show living properties outside a cell

• They can be crystallized

• Viruses can replicate inside the host but cannot grow

• They do not have metabolic machinery

Classification of Viruses

The viruses are classified based on their genetic material into two groups as per the Baltimore classification:

1. DNA virus- These viruses have DNA as their genetic material. E.g., Herpesviruses
2. RNA virus- These viruses have RNA as their genetic material. E.g., Reoviruses

However, these groups further form sub-groups to classify the virus.

Viroids: Although viruses are regarded as the smallest infectious microorganism, some even smaller rod-shaped agents than viruses exist, which usually infect plants. They are known as viroids. Unlike a virus, they do not even have a viral coat. They are just made of plain, naked RNA, which is very small, circular, and single-stranded. Such an RNA molecule does not even have protein-coding genes.

Even though they are made of just RNA, they are not digested because of a very tight secondary structure that is caused by extensive internal base pairing. The viroids are infectious to the plants. They are known to be causing several diseases in the plants, such as citrus exocortis, potato spindle tuber, and cucumber pale fruit disease.

Difference between virion and viroid

1. The virion is the extracellular phase of a virus that infects the host cell. It does not have the ability to grow and multiply in any artificial media as they are metabolically inert. However, viroids are not viruses; they are other agents which infect the plants.
2. The virion has genetic material enveloped in the protein case like a virus, but viroid does not have a protein case; they are naked RNA.
3. The viroids are made of plain, naked RNA, which is very small, circular, and single-stranded, which is different from the virions.
4. The viroids usually only infect plants, while the virions can infect Archaea, Bacteria and Eukarya, including animals and plants.

Conclusion

Viruses and viroids play a crucial role in evolutionary studies of the origin and development of life. And studying them increases our understanding of this most mysterious phenomenon called life on earth. Even though they are dangerous and can cause deadly diseases, they are quite a fascinating study. Several studies and books have been published on the subject of viruses and viroids.