Viruses and Viroids

Viruses and viroids

 Viruses and viroids are studied under microbiology. They are a part of the micro-ecosystem but impact our world in catastrophic ways. Some of the world’s greatest tragedies have occurred because of these minuscule organisms. Therefore, we must learn about these invisible agents of disaster as much as possible.

Virus

In basic terms, a virus is made of a genetic core, either DNA or RNA, and this core is surrounded by a protein encasing. They cannot replicate without the help of a host. They take over the host cell and use the cell’s components for replication. This often results in the destruction of the host cell. This is why viruses are parasitic and harm the health of their hosts.

There are viruses everywhere in the world. And their population is 10 times that of bacteria.

Size of viruses

  • Viruses are submicroscopic organisms. That means they are so small that they cannot be seen under a microscope. Their proper detection and imaging were only possible after the invention of the electron microscope

  • Viruses are quite a bit smaller than bacteria

Structure of viruses

  • Viruses exist in between living and non-living worlds. This is because they contain the protein materials, namely RNA or DNA, necessary in all living organisms but lack the materials necessary to interpret these sequences. This is why viruses cannot replicate on their own or synthesize materials on their own. They make use of the advanced cellular machinery of their hosts for these purposes

  • Viruses have a very basic structure. They contain nucleic acid in the form of either RNA or DNA inside a protein casing in the form of capsid (small units arranged in a certain way). The capsid casing prevents the destruction of the virus by the nucleases or host cell enzymes

  • Sometimes viruses also have another layer of covering known as an envelope. This layer is usually made after the virus has had access to some host cell

  • The nucleic acid, i.e. the DNA or the RNA, of the virus is its entire genome. This means that their genetic material is contained within this sequence of proteins

  • The genomes of viruses are typically small in size because they contain only the information needed for the capsid proteins, some enzymes and proteins necessary for replication

  • A virus has two forms- intracellular and extracellular. The extracellular form of a virus is capable of causing infection and is called a virion. Virions are fully assembled viruses, i.e. they have a genome and a capsid protein coat

Viroids

Viroids are also non-living agents of diseases. However, they are quite different from viruses. Viroids cause diseases in plants and are exclusively found in angiosperms or plants which produce flowers and seeds enclosed in a carpel. They do not have a protein covering and contain only a single strand of RNA. Nevertheless, they can propagate like viruses at the cost of the host organism.

Transmission

Viroids can be transmitted by aphids, by contact and exposure through mechanical means such as damaged tissue. For example, the potato tuber spindle disease was carried to other potato plants when potatoes were cut by knives carrying the viroid.

Replication

  • Viroids replicate in the chloroplasts or nucleus of plant cells

  • First, they extract the enzyme RNA polymerase II, an enzyme that plays a significant role in synthesising messenger RNA

  • This enzyme is then used for replicating the RNA of the viroid, the original viroid particle’s RNA acting as a template

Intrahost motility

  • Viroids do not move from one host to another on their own

  • They lack any proteins that help in propagation

  • Instead, they rely completely on the host or other mechanical means

  • Viroids are infectious, even if they cannot be transmitted on their own from one host to another. This is because they can replicate using the host’s cells to the detriment of the host plant. They have been known to cause devastating destruction to crops

Viruses and viroids

Both viruses and viroids form a large part of the pathogens found in the world today. Both viruses and viroids are infectious. Although they may share certain characteristics, they are not the same thing. Viruses and viroids are dissimilar in several ways.

Difference Between Virion and Viroid

  • A virion is an extracellular virus particle, so it may contain RNA or DNA strands that have the codes for further protein synthesis. Viroid genome, on the other hand, just contains a small, singular, and circular strand of RNA

  • Viroids do not produce movement proteins. Virions can produce movement proteins that help them move from one host to another

  • In a virion, the genome is contained in a capsid coat. Viroid is just made up of single-stranded RNA molecules and do not possess a capsid coat 

  • Viroids and virions differ in size, too. Viroids are significantly smaller than virions

  • Virions infect almost all living organisms, whereas viroids infect only plants

  • Viroids are subviral particles and are considered more primitive in their structure than viruses

These are some dissimilarities between viruses and viroids. However, they are both pathogens, and so they share some similarities too. The similarities between viruses and viroids are discussed in the next session.

Similarities between viruses and viroids

  • Both viruses and viroids cause infectious diseases

  • They are both submicroscopic particles

  • Some viruses have single-stranded RNA similar to viroids

  • Both need a host to replicate

  • Some viruses infect plants, as do all viroids

  • Antiviral medication is needed for the removal of diseases caused by both. They both are unaffected by antibiotic medicines

  • Some plant RNA viruses and all viroids require a double Stranded RNA intermediary for propagation

 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.  

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