Cell Biology
Subviral particles, such as prions and viroids, are smaller and simpler than viruses. Prions are pathogenic, misfolded proteins that can trigger normal, healthy proteins to misfold similarly, often converting alpha-helices into beta-pleated sheets. These misfolded proteins accumulate, leading to cell function deterioration and diseases like bovine spongiform encephalopathy (mad cow disease), Creutzfeldt-Jakob disease, and fatal familial insomnia. Prion diseases are rare and difficult to contract, but can sometimes be hereditary due to gene mutations causing protein misfolding.
Viroids, on the other hand, are plant-infecting pathogens composed of small circular, single-stranded RNA. They can reproduce only inside host cells and lack a capsid or outer envelope. Viroids do not encode any proteins; rather, they cause disease by silencing certain genes in the plant cell genome, leading to metabolic issues and structural damage. The hepatitis D virus (HDV), a viroid-like pathogen, can cause liver disease in humans but only in the presence of the hepatitis B virus (HBV).
Lesson Outline
<ul> <li>Introduction to Subviral Particles (Prions and Viroids)</li> <ul> <li>Prions and viroids are smaller and simpler than viruses</li> </ul> <li>Prions</li> <ul> <li>Pathogenic, misfolded proteins</li> <li>Resemble a beta-pleated sheet structure</li> <li>Prions trigger normal proteins to misfold in the same way</li> <li>Examples of prion diseases</li> <ul> <li>Bovine spongiform encephalopathy (Mad cow disease)</li> <li>Creutzfeldt-Jakob disease</li> <li>Fatal familial insomnia</li> </ul> <li>Hereditary and rare nature of prion diseases</li> </ul> <li>Viroids</li> <ul> <li>Plant-infecting pathogens</li> <li>Composed of small circular, single-stranded RNA</li> <li>Reproduce inside host cells but don't have a capsid or outer envelope</li> <li>Viroids cause disease by silencing certain genes in the plant cell genome</li> <li>Gene silencing leads to metabolic issues and structural damage in plants</li> </ul> <li>Hepatitis D Virus</li> <ul> <li>Viroid-like pathogen that can cause disease in humans</li> <li>Requires the presence of the Hepatitis B virus to complete its life cycle</li> <li>HBV and HDV combination can cause liver disease</li> <li>People can become infected with both hepatitis B virus and hepatitis D virus</li> </ul> </ul>
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FAQs
Subviral particles are infectious agents that are smaller and simpler than viruses. They lack some of the key components of viruses, such as a protein capsid or a complete genome. The two main types of subviral particles are prions, which are misfolded proteins, and viroids, which consist of single-stranded circular RNA molecules. Both types of particles differ from viruses: prions do not contain any nucleic acids, and viroids do not have a protein coat.
Prions cause diseases by inducing the conversion of normal cellular proteins into the abnormal prion form. This process is facilitated by protein misfolding, wherein the abnormal prion protein adopts a different three-dimensional structure compared to the healthy protein. This misfolded protein accumulates in the brain, causing cell damage and ultimately leading to the clinical signs of diseases such as Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle.
Viroids are small, single-stranded circular RNA molecules that do not have a protein coat. Their RNA structure usually forms a highly base-paired, rod-like shape with some internal loops. Viroids replicate within host cells by using the host's RNA polymerase, which recognizes and binds to the specific sequences within the viroid RNA. The host enzymes then synthesize new viroid RNA molecules through a rolling circle mechanism, leading to the formation of multimeric RNA intermediates that are later processed into individual viroid molecules.
The hepatitis D virus (HDV) is unique among human viruses as it requires the presence of another virus, hepatitis B virus (HBV), to infect and cause disease in humans. HDV contains a small, single-stranded, circular RNA genome that is structurally similar to viroids. However, unlike viroids, HDV also encodes a protein called the delta antigen, which is essential for its replication. The HDV RNA by itself is incapable of causing an infection, but when it is packaged with HBV surface proteins, forming a hybrid virus, it can infect liver cells and cause hepatitis D in humans.
Viroids can cause a wide range of plant diseases by interfering with normal host cell functions. One of the suggested mechanisms for viroid pathogenicity is through gene silencing, a natural defense mechanism used by plants against invading nucleic acids. When a viroid infects a plant, its RNA can be recognized as "foreign" and subsequently processed into small interfering RNAs (siRNAs). These siRNAs can trigger the degradation of any cellular RNA molecules with complementary sequences, causing the silencing of specific host genes. This disruption of gene expression can lead to the development of disease symptoms in infected plants.
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