Difference Between Viruses and Viroids

Viruses are microorganisms that contain genetic material either in the form of DNA or RNA. Viruses use the machinery of the host to multiply because they do not have their own machinery. This is why they are also called acellular organisms (organisms without any cellular machinery). Viruses are classified as microscopic infectious agents. They are widely known to cause many deadly diseases like HIV-AIDS, Ebola, Swine flu, etc. Viruses have been a major reason for the widespread of many infections and illnesses since time immemorial. In human history, viruses are among many types of infectious microorganisms that are known for spreading deadly outbreaks. Some of the outbreaks caused by viruses include the Ebola virus outbreak in 2014 in West Africa, the Swine flu outbreak in 2019, and the all-known pandemic caused by the Coronavirus, also known as the Covid-19 pandemic.

For medical professionals and scientists, viruses have always been a headache. Many research groups throughout the world are continuously researching vaccines against viruses, and they have always proved themselves as scientist's wily foes. While we know many things about viruses, such as their mechanics of protein synthesis and their basic cellular processes, there is still a huge part unknown about them.

In 1886, a German chemist and agricultural researcher, Adolf Mayer, published a research paper titled "Concerning the Mosaic Disease of Tobacco." In this paper, he detailed all his findings of the extensive research he did on tobacco plants affected by the disease. The disease caused the development of dark green, yellow, and gray splotches on the leaves, which resulted in the leaves' premature breaking. According to his study, Mayor found out that when the diseased leaf breaks, it is crushed, and the liquid that oozes out of the leaf goes to the healthy leaf. When the diseased liquid reaches the veins of the healthy leaves, they also start showing symptoms of diseased leaves, such as discoloration and speckling. From this observation, Mayer concluded that something is present in the leafy liquid of the diseased plant that results in the development of disease in the healthy plant. Something is transferred from diseased to the healthy plant.

To prove his theory correct, Mayer started planning more tangible experiments in his lab to identify the cause of the disease. He wanted to isolate the disease-causing factor from the leaves. He started working on identifying and cultivating the disease-causing pathogen using the principles of Robert Koch. Robert Koch is a well-known scientist in pathology, and he discovered the bacteria that cause tuberculosis. After isolating the disease-causing substance from the diseased leaf, Mayer started identifying them under the microscope. However, no results showed up, and his work remained unsuccessful.

Later, another scientist, Ivanovsky, used a Chamberland filter for the same purpose as Adolf Mayer. The Chamberland filter was used to isolate bacteria and related pathogens from the liquid. According to his paper published in 1972, he used the filtrate to infect the healthy leaf, and to his surprise, the liquid filtrate was contagious, and the healthy plant got the disease. After his work, another question arose about what is this particle that can even cross the Chamberland filter.

In 1892, Dmitri Iosifovich Ivanovsky conducted an experiment with a Chamberland filter and concluded that bacteria or a soluble poison might be responsible for the infection in tobacco plants. Later in 1898, Martinus Beijerinck discovered that the pathogen causing tobacco mosaic disease is not a bacteria but, instead, it is something else. He called it "contagion vivum fluidum," which means contagious living fluid. Beijerinck worked in collaboration with Ivanovsky and called the fluid a virus, a short form of a Latin word that means liquid poison. Despite the fact that the tests of Mayer, Ivanovsky, Beijerinck, and others that came after only suggested the presence of viruses, it would be several more decades before anyone really observed a virus.

Viroids are infectious pathogens with small single-stranded, circular RNAs. They are different from viruses because they do not have a protein coat. All the viroids known to date are inhabitants of flowering plants, mostly angiosperms, and most of them cause diseases. The first viroids were discovered in the 1970s, following the discoveries of the "subvisible" microorganisms by Antonie van Leeuwenhoek in 1675 and the smaller lifelike entities by Dmitri Iosifovich Ivanovsky and Martinus Beijerinck in 1892-1898. The International Committee on Taxonomy of Viruses recognized the distinctive characteristics of viroids by designating a new order of subviral agents. Theodor Otto Diener, a plant pathologist at the U.S. Department of Agriculture's Research Center in Beltsville, Maryland, discovered, initially molecularly described, and named the first recognized viroid, the pathogenic agent of the potato spindle tuber disease, in 1971. The shorthand for this virus is PSTVd or potato spindle tuber virus. Shortly after, the Citrus exports viroid (CEVd) was founded and combined, and knowledge of PSTVd and CEVd shaped the notion of the viroid.

Viruses have nucleic acid, but they do not contain any protein-coding DNA. In the host system, RNA polymerase is used for synthesizing mRNA (messenger RNA) from DNA. This enzyme is utilized by the virus for its own replication through the rolling circle mechanism. Using this method of replication, viruses synthesize new RNA molecules using the RNA of the viroid as a template. The replication intermediates are larger in size. In order to cleave and ligate them, viroids utilize the catalytic feature of the ribozyme. Ribozymes perform self-cleavage and ligation of unit-size genomes from those intermediates.

The original theory put forth by Diener in 1989 that viroids might be "living relics" from the widely accepted, prehistoric, and non-cellular RNA world has since been supported by others. It is currently believed that viroids and other components that resemble viroids may have originated from this recently discovered kind of retrotransposon following the discovery of retro zymes.

Viroids exclusively infect plants, and infectious viroids can spread from one plant to another by contact between leaves, by cross-contamination after mechanical damage to plants caused by horticultural or agricultural activities, or by aphids. In plant cells, the viroid uses an RNA-based mechanism that takes place in either the nucleus or chloroplast of the plant cell to multiply. To initiate the process of termination, viroids utilize the host cell's RNA polymerase II for the catalysis of the rolling circle mechanism of replication.

In contrast to plant viruses that make movement proteins, viroids are completely passive and rely only on the host, making them helpful in the investigation of plant RNA kinetics.

It is still a mystery how viroids can cause disease and symptoms in plant cells when they don't produce any protein products inside their sequences. There is evidence that the process involves RNA silencing. Firstly, modifications to a viroid's genome can significantly influence how virulent it is, reflecting the fact that any created siRNAs would have a lower base pairing complement with the target messenger RNA. Secondly, siRNAs with sequences from viroid genomes have been identified in infected plants. Finally, all relevant viroid-like symptoms are developed by transgenic expression of the potato spindle tuber viroid's noninfectious hpRNA. This suggests that a dicer enzyme targets double-stranded intermediate RNA produced by viroids during their replication and cleaves it into siRNAs that are then loaded onto the RNA-induced silencing complex. The classic viroid symptoms are brought on by the induction of translation degradation or inhibition of translation because the viroid siRNAs contain sequences capable of complementary base pairing with the plant's own messenger RNAs.

Retroviroids, which are RNAs with a DNA homologue, include retroviroids and retroviroid-like components. These creatures, which are closely related to the family of viruses known as "carnation tiny viroid-like RNA," are believed to be most peculiar to the carnation, Dianthus caryophyllus (CarSV RNA). These substances have a connection to double-strand break repair and may serve as a homologous substrate for recombination. Since reverse transcriptase, which is encoded by retroviruses, produces homologous DNA, these components are known as retroviroids.

Difference between virus and viroids Viruses and viroids are both tiny infectious agents, but they differ in their structures and modes of replication.

Viruses are small infectious agents that cannot replicate on their own. They consist of genetic material (either DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an outer envelope made of lipids. Viruses can only reproduce by infecting a host cell and using the host cell's machinery to make copies of themselves. Once the host cell is infected, the virus will take over the cell's metabolic processes, making the host cell produce more viruses. This eventually causes the host cell to burst, releasing new viruses into the environment.

Viroids, on the other hand, are much simpler than viruses. They consist of a small circular RNA molecule without any protective coat or envelope. Viroids do not encode any proteins and cannot replicate on their own. Instead, they rely on the host plant cells' RNA polymerase enzyme to replicate. Viroids do not cause the host cells to burst, but instead, they interfere with the host plant's normal gene expression, causing disease symptoms. Viroids infect only plants and do not affect animals.

One of the most significant differences between viruses and viroids is their size. Viruses are much larger than viroids and have a more complex structure. Viroids are about ten times smaller than viruses, and their structure is much simpler. Another difference is the type of genetic material they contain. While viruses can contain either DNA or RNA, viroids only have RNA.