Normal cellular protein PrP^C^ ( = cellular) exists in all mammals, especially neurons. It has a particular3D fold with lots of alpha helices. The prion form PrP^Sc^ (Sc = scrapie, the sheep disease where first found) has the same amino acid sequence but folds differently—more beta sheets.
HOW the infection spreads:
Contact: PrP^Sc^ physically touches PrP^C^
Conformational conversion: PrP^Sc^ acts as a template, forcing PrP^C^ to refold into PrP^Sc^
Aggregation: Multiple PrP^Sc^ molecules clump together into insoluble fibrils
Amplification: Each new PrP^Sc^ can convert more PrP^C^—exponential growth
WHY this is devastating:
The beta-sheet aggregates are resistant to proteases (enzymes that break down proteins)
Cannot be killed by heat, UV formaldehyde—standard sterilization fails
Accumulate in brain tissue → holes in brain tissue → spongiform encephalopathy
No immune response (body sees it as "self" protein, just misfolded)
Viroids hijack the plant's own machinery. The viroid RNA itself is the pathogen—it forms complex secondary structures (extensive intramolecular base-pairing → rod-like shape) that let it be recognized and processed by host enzymes. Crucially, viroids do not encode any proteins; they carry information purely as an RNA sequence and structure.
HOW cleavage happens depends on the viroid family (important nuance):
Pospiviroidae (e.g., PSTVd): replicate in the nucleus, and their multimeric RNA is cleaved by host RNase enzymes. These viroids have no ribozyme of their own.
Avsunviroidae (e.g., ASBVd, avocado sunblotch viroid): replicate in the chloroplast, and their RNA encodes self-cleaving hammerhead ribozyme motifs—the viroid RNA cleaves itself.
HOW viroids replicate:
Entry: Through wounds, pollen, seeds, contaminated tools
Trafficking: Viroid RNA moves cell-to-cell via plasmodesmata, long-distance via phloem
Replication: Host RNA polymerase (Pol II in nucleus for Pospiviroidae; a chloroplast polymerase for Avsunviroidae) mistakes viroid RNA for a template → synthesizes complementary RNA strand
Rolling circle: Complementary strand serves as template → produces multiple copies of viroid linked in a chain → cleaved (host RNase in Pospiviroidae; self-cleaving ribozyme in Avsunviroidae) → circularizes
Host RNase (Pospiviroidae) OR self-encoded ribozyme (Avsunviroidae)
Size
~250 amino acids (~28 kDa)
200-400 nucleotides (~110 kDa)
Transmission
Ingestion, surgery, hereditary
Mechanical, seeds, pollen
Sterilization
Resistant to standard methods
Destroyed by autoclaving
Disease examples
Kuru, CJD, BSE, scrapie
PSTVd, CCCVd, ASBVd
Key similarity: Both subvert host machinery without encoding their own enzymes. Both are "information parasites"—prion's information is a shape, viroid's is an RNA sequence.
Recall Explain to a 12-year-old
Imagine you have two weird infections:
Prion: You know how proteins are like folded origami? Each protein has a correct fold. A prion is a protein folded wrong—like someone made the paper crane backwards. The creepy part? If the wrong-folded protein touches a right-folded one, it forces it to fold wrong too! It's like zombie origami—one bad fold turns all the others bad. Your brain is full of these proteins, so if they all fold wrong, your brain gets holes in it and stops working. And because there's no virus or bacteria, just a bad shape, your immune system can't even see it. Super scary.
Viroid: Plants have machinery to read DNA and make stuff. A viroid is a tiny loop of RNA (like half of DNA) that sneaks into plant cells. The plant's machinery is dumb—it sees the RNA loop and thinks "oh, I should copy this!" So it copies the viroid over and over into one long chain. Then that long chain has to be cut into single loops. In most viroids the plant's own scissors-protein (an RNase) does the cutting. But in one special family, the viroid RNA has a built-in cutting trick (a ribozyme) and snips itself! The viroid doesn't even make proteins—it just exists and messes up the plant's normal work, like a fake worksheet jamming the copy machine.
Why they matter: They prove infection doesn't need all the stuff we thought it needed. Prions = no DNA/RNA. Viroids = no protein shell. Life is weirder than textbooks say!
Protein Folding and Misfolding Diseases - prions as extreme case of conformational disease
RNA Structure and Catalytic RNA - viroid ribozyme activity (Avsunviroidae)
Viral Structure and Classification - contrast with viruses
Plant Pathology - viroids as agricultural concern
Neurodegenerative Diseases - prion diseases among TSEs
Horizontal Gene Transfer - viroid evolutionary origins debated
Sterilization and Disinfection Methods - prion resistance challenge
RNA Interference and Gene Silencing - viroids interact with plant RNAi
#flashcards/biology
What is a prion and what makes it infectious? :: A prion is a misfolded protein (PrPSc) that causes disease by converting normal proteins (PrPC) to the same misfolded shape. It's infectious despite having no genetic material because the misfolded conformation itself acts as the infectious information.
Why can't standard sterilization methods destroy prions?
Prions are resistant to heat, UV radiation, and chemical disinfectants because the misfolded beta-sheet structure is extremely stable and resistant to protease degradation. Effective destruction requires extended autoclaving (134°C for 18min) or harsh chemicals like sodium hydroxide.
What is a viroid?
A viroid is the smallest known infectious agent—a small circular single-stranded RNA molecule (200-400 nucleotides) with no protein coat that infects plants by hijacking host machinery.
How do viroids replicate without encoding any proteins?
Viroids use the host plant's RNA polymerase (Pol II in the nucleus for Pospiviroidae; a chloroplast polymerase for Avsunviroidae), which mistakes the viroid RNA for a template. Through rolling circle replication, complementary strands are synthesized, then cleaved and circularized to produce new viroid copies.
Which viroids have ribozymes and which don't?
Only the Avsunviroidae family (e.g., ASBVd) encodes self-cleaving hammerhead ribozymes in their own RNA. The Pospiviroidae family (e.g., PSTVd, CCCVd) has NO ribozyme—their concatenated copies are cut by host RNase enzymes.
Is the ribozyme that cleaves viroid RNA supplied by the plant cell?
No. Plant cells do not provide endogenous ribozymes for this. Self-cleaving hammerhead ribozymes are encoded by the viroid itself, and only in the Avsunviroidae family. Pospiviroidae rely on host protein RNase enzymes for cleavage.
What are the key differences between prions and viroids?
Prions are infectious proteins with no genetic material that affect mammals through protein misfolding. Viroids are naked RNA molecules that infect plants by hijacking transcription machinery. Prions spread by conformational conversion; viroids replicate via host polymerase.
What disease did kuru teach us about prion transmission?
Kuru demonstrated that prions could be transmitted through cannibalistic practices (eating infected brain tissue) and could have extremely long incubation periods (10-50 years). The disease's decline after cessation of cannibalism confirmed the dietary transmission route.
Why are viroid diseases slow to develop?
Viroids don't directly kill cells or produce toxins. Disease develops slowly because pathology results from chronic disruption of gene regulation, interference with RNA processing, and sustained activation of plant stress responses over months to years.
What is the rolling circle mechanism in viroid replication?
In rolling circle replication, the polymerase uses the circular viroid complementary strand as a template and proceeds multiple times around the circle without releasing, producing a concatenated chain of multiple viroid copies that are then cleaved and circularized.
The immune system doesn't recognize prions as foreign because PrPSc has the same amino acid sequence as the normal PrPC protein—it's just misfolded. The body sees it as "self" and doesn't mount an immune response.
How did BSE (mad cow disease) cross to humans?
BSE prions accumulated in cattle nervous tissue after cows were fed prion-contaminated meat-and-bone meal. When humans consumed infected beef (especially CNS tissue), the cattle prions could convert human PrPC to PrPSc, causing variant CJD despite the species barrier making conversion inefficient.
Dekho, yaha basic idea ye hai ki hum sochte the ki infection failane ke liye kuch to genetic material chahiye—DNA ya RNA—jaise virus mein hota hai. Lekin prions ne ye rule tod diya! Prion sirf ek misfolded protein hai, matlab ek normal protein jo galat shape mein fold ho gaya. Iske paas na DNA hai, na RNA. Fir bhi ye infectious kaise hai? Kyunki jab ye misfolded protein (PrP-Sc) tumhare body ke normal proteins (PrP-C) ko touch karta hai, to wo unhe bhi apne jaise galat shape mein fold hone pe majboor kar deta hai. Ek se do, do se char—ye ek chain reaction ban jaata hai. Same amino acid sequence, bas fold alag—yahi "infectious information" hai.
Ab why-it-matters wali baat: ye misfolded shape mein zyada beta sheets hote hain jo bahut strong aur stable hote hain. Isliye ye proteases (jo enzymes normally proteins ko todte hain), heat, UV, aur normal sterilization se bhi nahi marte! Aur kyunki body inhe apna hi "self" protein samajhti hai (bas galat folded), immune system inpe attack hi nahi karta. Result? Ye brain mein jama hote jaate hain, tissue mein holes ban jaate hain (spongiform encephalopathy), aur neurodegeneration ho jaata hai. Ye math wala part—exponential growth—yahi explain karta hai ki incubation itna lamba (10-50 saal!) kyun hota hai lekin ek baar takeoff hone ke baad disease tezi se badhti hai.
Real-life examples se ye clear hota hai—Kuru disease Papua New Guinea ke Fore logo mein cannibalism se faili, aur Mad Cow Disease (BSE) infected beef khane se humans mein vCJD ban gayi. Ye examples yaad rakhna kyunki exam mein "transmission" aur "species barrier" ke concepts important hain—prion cross-species isliye jump kar paate hain kyunki PrP protein saare mammals mein kaafi similar (conserved) hota hai. Basically, prions humein sikhate hain ki sometimes shape hi information hoti hai—genes ki zaroorat hi nahi!