Prions, Definitions, Discovery, Types, Structure and Mechanism
Prions: This article aims to provide an overview of prions, including their definition, types, characteristics, structure, mechanism, and Differences from viruses.
Krati Saraswat4 Jun, 2025
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Prions are infectious agents primarily composed of abnormally folded proteins. Unlike other pathogens, such as bacteria and viruses, prions lack nucleic acids (DNA or RNA), making them unique in the spectrum of infectious agents. They cause neurodegenerative diseases by triggering normal proteins in the brain to misfold, leading to severe neurological damage. Understanding prions is vital for the NEET exam , and their study is crucial for the NEET biology syllabus , as it highlights the role of proteins in health and disease while providing insights into molecular biology and pathogenesis.
Definition of Prions
Prions are infectious agents composed entirely of protein. Unlike bacteria , viruses, or fungi , prions lack nucleic acids ( DNA or RNA ) and are responsible for a group of neurodegenerative diseases known as transmissible spongiform encephalopathies (TSEs). These diseases affect the brain and nervous system, leading to severe neurological dysfunction and ultimately death. Prions induce abnormal folding of specific normal cellular proteins, particularly those found in the brain, resulting in brain damage and characteristic symptoms.Discovery of Prions
Prions were first identified in the 1980s by Stanley Prusiner, who coined the term "prion," derived from "proteinaceous infectious particle." His groundbreaking research demonstrated that prions could cause diseases without any genetic material, fundamentally changing our understanding of infection and disease.Structure of Prions
Prions are composed primarily of misfolded proteins known as PrP (prion protein). The normal form of this protein, designated PrP^C (cellular prion protein), is harmless and primarily found in the brain. When it misfolds into a pathogenic form (PrP^Sc, or scrapie prion protein), it becomes infectious. The abnormal PrP^Sc induces the normal PrP^C to convert into the abnormal form, leading to an accumulation of misfolded proteins.Characteristics of Prions
Prions possess several unique characteristics:- Lack of Genetic Material: Prions are unique among infectious agents as they contain no DNA or RNA.
- Protein Misfolding: The hallmark of prion disease is the conversion of normal proteins into their misfolded counterparts.
- Infectivity : Prions are capable of inducing a conformational change in normal proteins, thus propagating the disease without the need for nucleic acids.
- Resistance to Degradation : Prions are resistant to proteases and can persist in the environment for long periods, making them particularly difficult to eradicate.
Types of Prions
Prion diseases are classified based on their clinical features, pathology, and transmission routes. Here are some notable examples:Creutzfeldt-Jakob Disease (CJD)
Creutzfeldt-Jakob Disease is the most common prion disease in humans. It typically presents with rapidly progressive dementia, psychiatric symptoms, and neurological deficits, such as ataxia and myoclonus. CJD can occur in sporadic, familial, or acquired forms. The sporadic form is the most prevalent, while the acquired form can result from exposure to infected tissues, such as through surgical procedures or consumption of contaminated meat.Bovine Spongiform Encephalopathy (BSE)
Bovine Spongiform Encephalopathy, commonly known as "mad cow disease ," affects cattle and is linked to the consumption of feed contaminated with prion-infected materials. Infected animals exhibit neurological symptoms such as changes in behavior, lack of coordination, and sensitivity to touch. BSE has significant implications for public health, as it can be transmitted to humans through the consumption of infected beef, leading to variant CJD (vCJD).Kuru
Kuru is a prion disease historically observed among the Fore people of Papua New Guinea, where it was transmitted through ritualistic cannibalism. Symptoms include tremors, loss of coordination, and psychological changes. The practice of cannibalism was discontinued in the 1960s, leading to a decline in new cases, although the disease's effects can persist for generations due to its long incubation period.Fatal Familial Insomnia (FFI)
Fatal Familial Insomnia is a rare inherited prion disease characterized by progressively worsening insomnia, leading to severe sleep disturbances and neurological decline. Patients experience autonomic dysfunction and cognitive impairments, eventually resulting in death. FFI is caused by specific mutations in the prion protein gene (PRNP) and has a familial pattern of inheritance.Scrapie
Scrapie primarily affects sheep and goats, causing severe neurological symptoms such as itching, behavioral changes, and lack of coordination. It is transmitted through contact with infected animals or contaminated environments . Scrapie has been recognized for centuries and is notable for its long incubation period, making it difficult to control within flocks.Mechanism of Action of Prion
Prions cause disease through a unique mechanism that involves the conversion of normal prion proteins (PrP^C) into misfolded, pathogenic forms (PrP^Sc) . This misfolding alters the normal structure of the proteins, leading to the aggregation of PrP^Sc . These aggregates accumulate in neural tissues, disrupting cellular function and leading to neurodegeneration. Importantly, prions do not contain any genetic material, which means they replicate by inducing conformational changes in other normal prion proteins, spreading the disease without the need for nucleic acids.Pathogenesis of Prion Diseases
The pathogenesis of prion diseases involves a series of events:- Infection: The introduction of PrP^Sc into a host, typically through contaminated food or medical procedures.
- Conversion: The normal prion proteins in the host undergo conformational changes, becoming misfolded.
- Aggregation : Misfolded proteins aggregate, forming insoluble plaques in the brain and other tissues.
- Neurotoxicity: The accumulation of these aggregates disrupts normal cellular functions, leading to neuronal death and the characteristic spongiform changes in brain tissue.
Transmission of Prions
Prions can be transmitted through several routes, including:- Consumption: Eating contaminated meat products, such as beef infected with BSE.
- Medical Procedures : Transmission can occur through surgical instruments contaminated with prion material or through blood transfusions from infected donors.
- Environmental Exposure: Prions can persist in the environment, contaminating soil and animal feed, leading to transmission among animals.
Symptoms of Prion Diseases
Following are the symptoms of Prion diseases:- Rapidly developing dementia,
- Difficulty walking and changes in gait,
- jerking movements of the muscles,
- Hallucinations.
Treatments for Prion Diseases
The human body is complex, and there are still many aspects that scientists have yet to fully understand. Prion diseases are among these complexities. Despite the advancements in technology today, an effective treatment for prion diseases has not yet been discovered.Difference Between Prions and Virus
The table below compares prions and viruses, emphasizing their composition, mechanisms of infection, and resistance to sterilization.| Difference Between Prions and Virus | ||
|---|---|---|
| Feature | Prions | Virus |
| Definition | Infectious proteins | Infectious particles containing genetic material (DNA or RNA) |
| Composition | Consist solely of protein without genetic material | Contain genetic material (either DNA or RNA) surrounded by a protein coat |
| Diseases Caused | Cause neurodegenerative diseases like Creutzfeldt-Jakob disease and mad cow disease | Cause various diseases, including the common cold, flu, and HIV |
| Mechanism of Infection | Convert normal proteins into misfolded, infectious forms, leading to accumulation in the brain | Infect host cells, using their machinery to replicate and produce more viruses |
| Size | Generally larger than viruses | Much smaller than prions |
| Range of Hosts | Primarily infect mammals | Can infect a wide range of organisms, including animals, plants, and bacteria |
| Resistance to Sterilization | Not destroyed by typical sterilization methods, making them difficult to control | Generally sensitive to heat and radiation, can be inactivated by standard sterilization methods |
| NEET Exam Important Links | |
|---|---|
| NEET Biology Syllabus | NEET Biology Diagrams |
| NEET Biology MCQ | NEET Biology Chapter wise Weightage |
| NEET Biology Notes | NEET Previous Year Question papers |
Prions FAQs
Q. What diseases do prions cause?
Ans. Prions are responsible for several neurodegenerative diseases, including Creutzfeldt-Jakob Disease (CJD), Bovine Spongiform Encephalopathy (BSE), commonly known as "mad cow disease," as well as Kuru and Fatal Familial Insomnia (FFI).
Q. Is prion a virus or bacteria?
Ans. Prions are neither viruses nor bacteria. They are infectious agents made solely of misfolded proteins and do not contain any nucleic acids, such as DNA or RNA, which sets them apart from traditional pathogens.
Q. Can prions be cured?
Ans. Currently, there is no cure for prion diseases. Treatment mainly focuses on managing symptoms and providing supportive care, as these diseases are progressive and ultimately fatal.
Q. How is a prion disease transmitted?
Ans. Prion diseases can be transmitted through several means, including the consumption of contaminated meat products, such as those from infected cattle, inherited genetic mutations, and medical procedures that involve contaminated instruments or tissues.
Q. Do prions have DNA or RNA?
Ans. No, prions do not have DNA or RNA. They consist entirely of abnormal protein forms, which allow them to induce misfolding of normal proteins in the brain.
Q. Who discovered prions?
Ans. Dr. Stanley B. Prusiner discovered prions in the 1980s and introduced the term "prion" to describe these infectious proteins. He was awarded the Nobel Prize in Physiology or Medicine in 1997 for his pioneering research on prions.
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