Explain prions and viroids — Hinglish notes

5.7.9 · Biology › Microbiology

What Are Prions?

The Biology of Prions

KYUN prions genes ke bina bhi infectious hote hain:

Normal cellular protein PrP^C^ ( = cellular) sabhi mammals mein, khaaskar neurons mein, paya jaata hai. Iska ek specific 3D fold hota hai jisme bahut saare alpha helices hote hain. Prion form PrP^Sc^ (Sc = scrapie, wo sheep disease jisme yeh pehli baar mila) ki same amino acid sequence hoti hai lekin yeh alag fold hota hai—zyada beta sheets hote hain.

KAISE infection spread hoti hai:

Contact: PrP^Sc^ physically PrP^C^ ko touch karta hai
Conformational conversion: PrP^Sc^ ek template ki tarah kaam karta hai, PrP^C^ ko PrP^Sc^ mein refold hone par majboor karta hai
Aggregation: Kaafi saare PrP^Sc^ molecules milke insoluble fibrils mein clump ho jaate hain
Amplification: Har naya PrP^Sc^ aur zyada PrP^C^ ko convert kar sakta hai—exponential growth

KYUN yeh itna devastating hai:

Beta-sheet aggregates proteases ke against resistant hote hain (wo enzymes jo proteins ko break down karte hain)
Heat, UV, formaldehyde se nahi maarte—standard sterilization fail ho jaata hai
Brain tissue mein accumulate hote hain → brain tissue mein holes → spongiform encephalopathy
Koi immune response nahi hota (body ise "self" protein maanti hai, bas misfolded)

Worked example Example 1: Kuru in the Fore People

Setup: Papua New Guinea ke Fore log funerary cannibalism practice karte the—maare gaye rishtedaaron ke dimag khate the respect ritual ki tarah. Kya hua:

Prions se infected person → rishtedaaron ne khaya → prions digestive tract mein gaye → bloodstream mein cross kiya → brain tak pahunche
Incubation: 10-50 saal
Symptoms: Coordination ka loss ("kuru" = kaanpna), dementia, symptom onset ke 1 saal ke andar maut
Epidemic: Hazaaron ki maut, mostly women aur children (jinhone brain tissue khaya)

KYUN yeh step? Prions digestion survive karte hain kyunki proteases misfolded beta-sheet structure ko break nahi kar sakti.

Resolution: Practice 1960s mein band hui → naye cases band ho gaye (dietary transmission confirm kiya) → kuru ab almost extinct hai.

Worked example Example 2: Mad Cow Disease (BSE) to vCJD

Setup: Cattle ko meat-and-bone meal khilaaya gaya jisme prion-infected sheep (scrapie) ya cattle remains the.

Events ka chain:

Cows Bovine Spongiform Encephalopathy (BSE) develop karte hain—prions nervous tissue mein accumulate hote hain
Humans infected beef khaate hain (khaaskar brain, spinal cord)
Prions species barrier cross karte hain → humans mein variant Creutzfeldt-Jakob Disease (vCJD) cause karte hain
Symptoms: Psychiatric changes, ataxia, dementia, maut (onset se average 13 mahine) KYUN cross-species jump kaam karta hai: PrP protein mammals mein highly conserved hai. Cow PrP^Sc^ human PrP^C^ ke itna similar hai ki misfolding induce kar sake.

KYUN yeh rare tha jab millions ne beef khaya: Species barrier = inefficient conversion. Zyaadatar prions human protein convert nahi karte. Lekin kaafi exposure + unlucky individuals → UK mein 178 deaths.

What Are Viroids?

The Biology of Viroids

KYUN viroids proteins ke bina infect kar sakte hain:

Viroids plant ki apni machinery ko hijack karte hain. Viroid RNA khud hi pathogen hai—yeh complex secondary structures banata hai (extensive intramolecular base-pairing → rod-like shape) jo isse host enzymes dwara recognize aur process hone deta hai. Importantly, viroids koi bhi protein encode nahi karte; ye information purely RNA sequence aur structure ke roop mein carry karte hain.

KAISE cleavage hoti hai yeh viroid family par depend karta hai (important nuance):

Pospiviroidae (e.g., PSTVd): nucleus mein replicate karte hain, aur inke multimeric RNA ko host RNase enzymes cleave karti hain. In viroids mein apna koi ribozyme nahi hota.
Avsunviroidae (e.g., ASBVd, avocado sunblotch viroid): chloroplast mein replicate karte hain, aur inki RNA self-cleaving hammerhead ribozyme motifs encode karti hai—viroid RNA khud ko khud cleave karta hai.

KAISE viroids replicate karte hain:

Entry: Wounds, pollen, seeds, contaminated tools ke zariye
Trafficking: Viroid RNA cell-to-cell plasmodesmata ke zariye, long-distance phloem ke zariye move karta hai
Replication: Host RNA polymerase (Pospiviroidae ke liye nucleus mein Pol II; Avsunviroidae ke liye ek chloroplast polymerase) viroid RNA ko template samajhne ki galti karta hai → complementary RNA strand synthesize karta hai
Rolling circle: Complementary strand template ki tarah kaam karta hai → ek chain mein linked kaafi saare viroid copies banata hai → cleave hota hai (Pospiviroidae mein host RNase; Avsunviroidae mein self-cleaving ribozyme) → circularize hota hai

Formula Viroid Replication: Rolling Circle Model

Given: Circular (+) strand viroid RNA cell mein enter karta hai Step 1 - Complementary strand synthesis: Host polymerase (+) strand ko template ki tarah read karta hai: $(+) viroid host Pol (-) complementary strand$

Step 2 - Rolling circle: $(-)$ strand template ki tarah kaam karta hai. Polymerase release kiye bina circle ke around kaafi baar jaata hai: $(-) strand host Pol (+) (+) \dots (concatenated)$

Step 3 - Cleavage: $(+) (+) \to (+) + (+) + (+)$ Cleavage agent family par depend karta hai: host RNase (Pospiviroidae) ya self-encoded hammerhead ribozyme (Avsunviroidae).

Step 4 - Circularization: Har linear (+) strand khud se ligate ho jaata hai: $Linear (+) ligase Circular (+)$

KYUN rolling circle? Output maximize karta hai—ek template re-initiating ke bina kaafi copies banata hai. Plasmids aur phages mein bhi common hai.

Worked example Example 3: Potato Spindle Tuber Viroid (PSTVd)

Pathogen: 359-nucleotide circular RNA (family Pospiviroidae — koi ribozyme nahi, host RNase se cleave hota hai) Host: Potato, tomato, doosri solanaceous crops

Symptoms:

Potatoes: Spindle-shaped tubers (elongated, pointed ends), cracked skin, reduced size
Tomatoes: Stunted growth, leaf epinasty (neeche ki taraf curling), fruit deformation Economic impact: Potato yields 50% tak reduce kar sakta hai

KYUN spindle-shape? Viroid auxin signaling disrupt karta hai → tuber development ke dauran abnormal cell elongation patterns.

Transmission: Vegetative propagation (infected seed tubers), mechanical tools, kabhi-kabhi pollen se.

Control: Certified virus-free seed stock use karo, cutting tools sterilize karo (viroids mild heat se resist karte hain lekin 121°C par 20min mein destroy ho jaate hain).

Comparing Prions and Viroids

Feature	Prion	Viroid
Composition	Sirf protein (PrP^Sc^)	Sirf RNA (circular ssRNA)
Genetic material	Koi nahi	RNA (lekin non-coding)
Host	Mammals	Plants
Replication mechanism	Host PrP^C^ ki conformational conversion	Host RNA polymerase + rolling circle
Cleavage of copies	N/A	Host RNase (Pospiviroidae) YA self-encoded ribozyme (Avsunviroidae)
Size	~250 amino acids (~28 kDa)	200-400 nucleotides (~110 kDa)
Transmission	Ingestion, surgery, hereditary	Mechanical, seeds, pollen
Sterilization	Standard methods se resistant	Autoclaving se destroy ho jaate hain
Disease examples	Kuru, CJD, BSE, scrapie	PSTVd, CCCVd, ASBVd

Key similarity: Dono apne enzymes encode kiye bina host machinery subvert karte hain. Dono "information parasites" hain—prion ki information ek shape hai, viroid ki ek RNA sequence.

Recall Ek 12-saal-ke bachche ko explain karo

Socho tumhare paas do weird infections hain: Prion: Tumhe pata hai proteins folded origami ki tarah hote hain? Har protein ka ek sahi fold hota hai. Ek prion ek aisa protein hai jo galat fold hua hai—jaise kisi ne paper crane ulta banaya. Creepy part? Agar galat-folded protein sahi-folded ek ko touch kare, toh wo use bhi galat fold hone par majboor karta hai! Yeh zombie origami ki tarah hai—ek bad fold saare doosron ko bad bana deta hai. Tumhara brain in proteins se bhara hai, toh agar sab galat fold ho jaayein, tumhare brain mein holes ho jaate hain aur woh kaam karna band kar deta hai. Aur kyunki koi virus ya bacteria nahi hai, sirf ek bad shape hai, tumhara immune system ise dekh bhi nahi sakta. Bahut scary.

Viroid: Plants mein machinery hoti hai DNA read karke kaam karne ke liye. Ek viroid RNA ka ek tiny loop hai (DNA ka aadha jaisa) jo plant cells mein ghus jaata hai. Plant ki machinery bewakoof hai—woh RNA loop dekhti hai aur sochti hai "oh, mujhe yeh copy karni chahiye!" Toh woh viroid ko baar baar copy karke ek lambi chain banaa deti hai. Phir us lambi chain ko single loops mein katna padta hai. Zyaadatar viroids mein plant ki apni scissors-protein (ek RNase) cutting karti hai. Lekin ek special family mein, viroid RNA mein ek built-in cutting trick (ek ribozyme) hoti hai aur woh khud ko snip karta hai! Viroid proteins bhi nahi banata—yeh bas exist karta hai aur plant ka normal kaam bigaad deta hai, jaise ek fake worksheet copy machine jam kar de.

Kyun ye matter karte hain: Ye prove karte hain ki infection ke liye wo saari cheezein zaroori nahi jo humne socha tha. Prions = koi DNA/RNA nahi. Viroids = koi protein shell nahi. Life textbooks se zyaada weird hai!

Connections

Protein Folding and Misfolding Diseases - prions conformational disease ka extreme case
RNA Structure and Catalytic RNA - viroid ribozyme activity (Avsunviroidae)
Viral Structure and Classification - viruses se contrast
Plant Pathology - viroids as agricultural concern
Neurodegenerative Diseases - prion diseases TSEs mein
Horizontal Gene Transfer - viroid evolutionary origins debated
Sterilization and Disinfection Methods - prion resistance challenge
RNA Interference and Gene Silencing - viroids plant RNAi se interact karte hain

#flashcards/biology

Prion kya hai aur ise infectious kya banata hai? :: Ek prion ek misfolded protein (PrPSc) hai jo disease cause karta hai normal proteins (PrPC) ko same misfolded shape mein convert karke. Yeh infectious hai koi genetic material na hone ke bawajood kyunki misfolded conformation khud hi infectious information ki tarah kaam karta hai.

Standard sterilization methods prions ko destroy kyun nahi kar sakti?

Prions heat, UV radiation, aur chemical disinfectants se resistant hote hain kyunki misfolded beta-sheet structure extremely stable hoti hai aur protease degradation se resistant hoti hai. Effective destruction ke liye extended autoclaving (134°C for 18min) ya harsh chemicals jaise sodium hydroxide chahiye.

Viroid kya hai?

Ek viroid sabse chhota jaana-maana infectious agent hai—ek small circular single-stranded RNA molecule (200-400 nucleotides) jo bina kisi protein coat ke plants ko infect karta hai host machinery hijack karke.

Viroids koi bhi protein encode kiye bina kaise replicate karte hain?

Viroids host plant ki RNA polymerase (Pospiviroidae ke liye nucleus mein Pol II; Avsunviroidae ke liye ek chloroplast polymerase) use karte hain, jo viroid RNA ko template samajhne ki galti karta hai. Rolling circle replication ke zariye complementary strands synthesize hoti hain, phir cleave aur circularize hoti hain naye viroid copies banane ke liye.

Kis viroid mein ribozyme hota hai aur kis mein nahi?

Sirf Avsunviroidae family (e.g., ASBVd) apni RNA mein self-cleaving hammerhead ribozymes encode karta hai. Pospiviroidae family (e.g., PSTVd, CCCVd) mein koi ribozyme NAHI hota—inke concatenated copies host RNase enzymes dwara kate jaate hain.

Kya viroid RNA ko cleave karne wala ribozyme plant cell supply karta hai?

Nahi. Plant cells is kaam ke liye endogenous ribozymes provide nahi karti. Self-cleaving hammerhead ribozymes viroid khud encode karta hai, aur sirf Avsunviroidae family mein. Pospiviroidae cleavage ke liye host protein RNase enzymes par rely karte hain.

Prions aur viroids ke beech key differences kya hain?

Prions infectious proteins hain jisme koi genetic material nahi hota aur jo protein misfolding ke zariye mammals ko affect karte hain. Viroids naked RNA molecules hain jo transcription machinery hijack karke plants ko infect karte hain. Prions conformational conversion se spread hote hain; viroids host polymerase ke zariye replicate karte hain.

Kuru ne prion transmission ke baare mein humein kya sikhaya?

Kuru ne demonstrate kiya ki prions cannibalistic practices (infected brain tissue khaane) ke zariye transmit ho sakte hain aur bahut lamba incubation period ho sakta hai (10-50 saal). Cannibalism band hone ke baad disease ka decline hona dietary transmission route confirm karta hai.

Viroid diseases slowly kyun develop hoti hain?

Viroids directly cells nahi maarte ya toxins produce nahi karte. Disease slowly develop hoti hai kyunki pathology gene regulation ke chronic disruption, RNA processing mein interference, aur months se saalon tak plant stress responses ke sustained activation se accumulate hoti hai.

Viroid replication mein rolling circle mechanism kya hai?

Rolling circle replication mein, polymerase circular viroid complementary strand ko template ki tarah use karta hai aur release kiye bina circle ke around kaafi baar ghoomta hai, jo ek concatenated chain mein kaafi saare viroid copies produce karta hai jo phir cleave aur circularize ho jaate hain.

Prion diseases immune response kyun trigger nahi kar sakti?

Immune system prions ko foreign recognize nahi karta kyunki PrPSc ki normal PrPC protein se same amino acid sequence hoti hai—yeh sirf misfolded hai. Body ise "self" maanta hai aur koi immune response mount nahi karta.

BSE (mad cow disease) humans mein kaise cross ki?

BSE prions cattle nervous tissue mein accumulate hue jab cows ko prion-contaminated meat-and-bone meal khilaaya gaya. Jab humans ne infected beef (khaaskar CNS tissue) khaya, cattle prions human PrPC ko PrPSc mein convert kar sake, variant CJD cause karte hue jabki species barrier conversion ko inefficient banata hai.