What are viroids give example?
Potato spindle tuber viroidPeach latent mosaic viroidViroid/Representative species
What is viroids answer?
Viroids are infectious pathogens that affect only plants, therefore are also called plant pathogens. Structurally, viroids are smaller than viruses and possess circular strands of ribonucleic acids (RNAs) with no protein coating.
What is the function of a viroid?
Viroids are noncoding RNAs that infect plants. In order to establish systemic infection, these RNAs must traffic from an initially infected host cell into neighboring cells and ultimately throughout a whole plant.
Are viroids microorganisms?
Microorganisms Found in the Environment Viroids are infectious circular single-stranded RNAs from 250 to 400 nucleotides long that replicate in their plant host and are transmitted from host to host through mechanical means, e.g., through a wound or through transmission of contaminated pollen or in ovules.
What disease causes viroids?
Potato spindle tuber is a disease caused by viroid in potato.
What is the structure of viroid?
Known viroids are single-stranded, covalently closed circular, as well as linear, RNA molecules with extensive regions of intramolecular complementarity; they exist in their native state as highly base-paired rods.
Where are viroids found?
Viroids are plant pathogens with economic importance. Viroid genomes are extremely small in size, only about 300 nucleotides. Viroids have been found in agricultural products, such as potatoes, tomatoes, apples, and coconuts.
What are the difference between virus and viroids?
Viroids are free RNA molecules of low molecular weight without any protein coat while viruses can have either RNA or DNA molecules encapsulated in a protein coat. Viroids are smaller in size than viruses. Viroids infect only plants whereas virus infects all types of organisms.
How are viroids transmitted?
Viroids may be spread via vegetative propagules, mechanical damage, seed, pollen, or biological vectors.
Do viroids have protein?
Because viroids have no protein coat, they lack attachment proteins and cannot recognize and penetrate healthy cells as can a true virus.
Who discovered viroids?
Dr. Theodor 0. DienerThe pathogen is called a viroid by its discoverer, Dr. Theodor 0. Diener, to distinguish it from a virus.
How many viroids are there?
To date, more than 30 species of viroid have been reported exclusively from angiosperms.
What are viroids simple?
viroid, an infectious particle smaller than any of the known viruses, an agent of certain plant diseases. The particle consists only of an extremely small circular RNA (ribonucleic acid) molecule, lacking the protein coat of a virus.
What is viroid biology class 11?
Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economic importance to humans varies widely. Viroid. Virus classification.
What are viroids topper?
Viroids are small, single-stranded circular RNA agents which infect plants.
What is meant by viroids in biology class 11?
Note: viroids are the smallest pathogens known to humans. They only infect the plant and cause diseases in them. They are not found to be infectious in humans or animals. They have single-stranded RNA and have an absence of the protein coat on them.
What is the pathogenicity of viroids?
Viroid primarily infects, replicates and induces serious disease in higher plants. The pathogenicity of viroid differs within the distinct host species. Viroids cause latent infection, and they may also induce chronic infections in their host plants.
When were viroids discovered?
A plant pathologist, Theodor Diener discovered viroids in the year 1971 . Till date, the mechanism of host infection by viroids is not clear.
What enzyme cleaves the RNA strand of a viroid into unit lengths?
Thus, the RNase enzyme cleaves the (+) RNA strand of viroid into unit viroid lengths. Then, the unit fragments of the (+) RNA strand molecule undergo ligation to form the (+) circular RNA.
What are the two enzymes needed for viroid replication?
RNA polymerase, an RNase and an RNA ligase are the enzymes needed for viroid replication. Symmetric and asymmetric replication are the two replicative mechanisms of viroid.
What was the first viroid discovered?
Potato spindle tuber viroids (PSTV) was the first viroid discovered by T.O. Diener and W.B. Raymer in 1967. PSTV caused massive damage to potatoes. Its discovery led to the existence of some infectious entities smaller than viruses.
What do the folded regions of the viroid do?
The folded regions of the viroid help in protecting the RNA against the action of cellular enzymes.
What is the role of a positive circular strand in RNA?
The positive circular strand will function as a template in producing new RNA via a rolling circle mechanism. Then, it initiates the synthesis of mRNA from the cellular DNA, and make a large linear multimeric (-) strand by using a viroid circular (+) RNA strand.
How are viroids transcribed?
The viroids are transcribed from a cellular DNA of the host cell complementary to viroid RNA. In the infected cell new DNA may be produced with the infecting viroid RNA which serves as template. This makes the assumption for the presence of reverse transcriptase i.e. RNA directed DNA polymerase. From this the viroid RNAs are synthesized.
Where did viroids come from?
It is supposed that viroids would have been originated from virus induced low molecular weight RNAs which later on adapted as autonomously replicating infectious entities. Therefore viroids provide the evidence that they are the degenerated virus entities.
How is avsunviroid replication self cleaved?
In Avsunviroid replication the long sense RNA is self cleaved by ribozyme activity. A negative circle is formed upon circularization of RNA. A second rolling circle event makes a long linear positive strand, which is again cleaved by the activity of ribozyme. Then the short viroid RNA is ligated to form the circular structure.
What enzymes are required for replication of viroids?
Three enzymes are required for replication of viroids e.g. RNA polymerase, RNase and RNA ligase. RNA polymerase 11 is involved in synthesis of mRNA from DNA. Using viroid’s RNA as template, this enzyme catalyzes the synthesis of new RNA by rolling circle mechanism.
How do viroids spread?
with pointed ends having numerous eyes and heavy brows. The viroids are contagious and spread mainly through mechanical injury/contact but also through pollen and true seeds from the infected plants. The control measures are the use of diseased free seeds, early roguing and avoiding cutting of potato tubers.
How many nucleotides are in a virod?
Viroids are small (200- 400 nucleotide long), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope arid are associated with certain plant diseases. Their replication strategy is similar to that of viruses as they are also obligate intracellular parasites.
Do viroids replicate?
There is no convincing evidence for the replication of viroid genome. It is likely that nucleic acid codes for an enzyme replicase which is essential for its replications. Possibly, the members of Avsunviroids replicate in chloroplasts, whereas that of Pospiviroids replicate inside the nucleus and nucleolus.
What are viroids made of?
Viroids are small infectious pathogens. They are composed solely of a short strand of circular, single-stranded RNA. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms, and most cause diseases, whose respective economic importance to humans varies widely. The first discoveries ...
When was Viroid discovered?
The first recognized viroid, the pathogenic agent of the potato spindle tuber disease, was discovered, initially molecularly characterized, and named by Theodor Otto Diener, plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, in 1971.
How do viroids induce symptoms?
There has long been uncertainty over how viroids induce symptoms in plants without encoding any protein products within their sequences. Evidence suggests that RNA silencing is involved in the process. First, changes to the viroid genome can dramatically alter its virulence. This reflects the fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants. Finally, transgenic expression of the noninfectious hpRNA of potato spindle tuber viroid develops all the corresponding viroid-like symptoms. This indicates that when viroids replicate via a double stranded intermediate RNA, they are targeted by a dicer enzyme and cleaved into siRNAs that are then loaded onto the RNA-induced silencing complex. The viroid siRNAs contain sequences capable of complementary base pairing with the plant's own messenger RNAs, and induction of degradation or inhibition of translation causes the classic viroid symptoms.
How does the viroid genome affect virulence?
First, changes to the viroid genome can dramatically alter its virulence. This reflects the fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants.
What is the mechanism of viroid reproduction?
The reproduction mechanism of a typical viroid. Leaf contact transmits the viroid. The viroid enters the cell via its plasmodesmata. RNA polymerase II catalyzes rolling-circle synthesis of new viroids.
How are viroids transmitted?
Viroid infections can be transmitted by aphids, by cross contamination following mechanical damage to plants as a result of horticultural or agricultural practices, or from plant to plant by leaf contact.
When was the first pathogen of a eukaryotic organism for which the complete molecular structure has?
determined the complete nucleotide sequence of potato spindle tuber viroid in 1978. PSTVd was the first pathogen of a eukaryotic organism for which the complete molecular structure has been established. Over thirty plant diseases have since been identified as viroid-, not virus-caused, as had been assumed.
What is a viroid?
In 1971, Theodor Diener, a pathologist working at the Agriculture Research Service, discovered an acellular particle that he named a viroid, meaning “virus-like.” Viroids consist only of a short strand of circular RNA capable of self-replication. The first viroid discovered was found to cause potato tuber spindle disease, which causes slower sprouting and various deformities in potato plants (see Figure 6.24 ). Like viruses, potato spindle tuber viroids (PSTVs) take control of the host machinery to replicate their RNA genome. Unlike viruses, viroids do not have a protein coat to protect their genetic information.
How many types of virusoids are there?
There are currently only five described types of virusoids and their associated helper viruses. The helper viruses are all from the family of Sobemoviruses. An example of a helper virus is the subterranean clover mottle virus, which has an associated virusoid packaged inside the viral capsid.
How many nucleotides are in a virusoid genome?
The virusoid genomes are small, only 220 to 388 nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate virusoid RNA. Virusoids belong to a larger group of infectious agents called satellite RNA s, which are similar pathogenic RNAs found in animals.
What type of RNA is used to infect crops?
A second type of pathogenic RNA that can infect commercially important agricultural crops are the virusoids, which are subviral particles best described as non–self-replicating ssRNAs. RNA replication of virusoid s is similar to that of viroids but, unlike viroids, virusoids require that the cell also be infected with a specific “helper” virus. There are currently only five described types of virusoids and their associated helper viruses. The helper viruses are all from the family of Sobemoviruses. An example of a helper virus is the subterranean clover mottle virus, which has an associated virusoid packaged inside the viral capsid. Once the helper virus enters the host cell, the virusoids are released and can be found free in plant cell cytoplasm, where they possess ribozyme activity. The helper virus undergoes typical viral replication independent of the activity of the virusoid. The virusoid genomes are small, only 220 to 388 nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate virusoid RNA.
What is the first virodote?
The first viroid discovered was found to cause potato tuber spindle disease, which causes slower sprouting and various deformities in potato plants (see Figure 6.24 ). Like viruses, potato spindle tuber viroids (PSTVs) take control of the host machinery to replicate their RNA genome.
What is the potato spindle viroid?
Figure 6.24 These potatoes have been infected by the potato spindle tuber viroid (PSTV), which is typically spread when infected knives are used to cut healthy potatoes, which are then planted. (credit: Pamela Roberts, University of Florida Institute of Food and Agricultural Sciences, USDA ARS)
What is the name of the disease caused by proteinaceous particles?
Prions. At one time, scientists believed that any infectious particle must contain DNA or RNA. Then, in 1982, Stanley Prusiner, a medical doctor studying scrapie (a fatal, degenerative disease in sheep) discovered that the disease was caused by proteinaceous infectious particles, or prions.
What is a Viroid?
In 1971, Theodour Diener, a pathologist working at the Agriculture Research Service, discovered an acellular particle that he named a viroid, meaning “virus-like.”. Viroids consist only of a short strand of circular RNA capable of self-replication.
How many types of virusoids are there?
There are currently only five described types of virusoids and their associated helper viruses. The helper viruses are all from the family of Sobemoviruses. An example of a helper virus is the subterranean clover mottle virus, which has an associated virusoid packaged inside the viral capsid.
How many nucleotides are in a virusoid genome?
The virusoid genomes are small, only 220 to 388 nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate virusoid RNA. Virusoids belong to a larger group of infectious agents called satellite RNA s, which are similar pathogenic RNAs found in animals.
What is the first virodote?
The first viroid discovered was found to cause potato tuber spindle disease, which causes slower sprouting and various deformities in potato plants (see Figure 6.23 ). Like viruses, potato spindle tuber viroids (PSTVs) take control of the host machinery to replicate their RNA genome.
What is the potato spindle virus?
These potatoes have been infected by the potato spindle tuber viroid (PSTV), which is typically spread when infected knives are used to cut healthy potatoes, which are then planted. (credit: Pamela Roberts, University of Florida Institute of Food and Agricultural Sciences, USDA ARS)
Why are prions so hard to destroy?
Prions are extremely difficult to destroy because they are resistant to heat, chemicals, and radiation. Even standard sterilization procedures do not ensure the destruction of these particles. Currently, there is no treatment or cure for TSE disease, and contaminated meats or infected animals must be handled according to federal guidelines to prevent transmission.
Overview
Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economic importance to humans varies widely.
The first discoveries of viroids in the 1970s triggered the historically third majo…
Taxonomy
As of 2005 :
• Family Pospiviroidae
• Family Avsunviroidae
Transmission and replication
Viroids only infect plants, and infectious viroids can be transmitted to new plant hosts by aphids, by cross contamination following mechanical damage to plants as a result of horticultural or agricultural practices, or from plant to plant by leaf contact. Upon infection, viroids replicate in the nucleus (Pospiviroidae) or chloroplasts (Avsunviroidae) of plant cells in three steps through an RNA-based mechanism. They require RNA polymerase II, a host cell enzyme normally associate…
RNA silencing
There has long been uncertainty over how viroids induce symptoms in plants without encoding any protein products within their sequences. Evidence suggests that RNA silencing is involved in the process. First, changes to the viroid genome can dramatically alter its virulence. This reflects the fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated fro…
Retroviroids
Retroviroids and retroviroid-like elements are viroids, which are RNA that have a DNA homologue. These entities are thought to be largely exclusive to the carnation, Dianthus caryophyllus, that are closely related to the family of viruses termed 'carnation small viroid-like RNA' (CarSV RNA). These elements may act as a homologous substrate upon which recombination may occur and are linked to double-stranded break repair. These elements are dubbed retroviroids as the homologo…
RNA world hypothesis
Diener's 1989 hypothesis had proposed that the unique properties of viroids make them more plausible macromolecules than introns, or other RNAs considered in the past as possible "living relics" of a hypothetical, pre-cellular RNA world. If so, viroids have assumed significance beyond plant virology for evolutionary theory, because their properties make them more plausible candidates than other RNAs to perform crucial steps in the evolution of life from inanimate matt…
Control
The development of tests based on ELISA, PCR, and nucleic acid hybridization has allowed for rapid and inexpensive detection of known viroids in biosecurity inspections, phytosanitary inspections, and quarantine. However, the ongoing discovery and evolution of new viroids makes such assays always incomplete.
History
In the 1920s, symptoms of a previously unknown potato disease were noticed in New York and New Jersey fields. Because tubers on affected plants become elongated and misshapen, they named it the potato spindle tuber disease.
The symptoms appeared on plants onto which pieces from affected plants had been budded—indicating that the disease was caused by a transmissible pathogenic agent. A fungus or