Explain the role of microRNA and RNA interference

3.5.12 · Biology › Mutations & Gene Regulation

What Are MicroRNAs?

Key properties:

Non-coding: miRNAs DNA se transcribe hote hain lekin kabhi protein mein translate nahi hote
Endogenous: Organism ke apne genome mein encoded hote hain (viral siRNAs ke unlike)
Sequence-specific: Har miRNA ka ek "seed region" hota hai (nucleotides 2-8) jo determine karta hai ki woh kaun si mRNAs ko target karega
Conserved: Kaafi miRNAs door-door species mein identical hote hain (e.g., let-7 worms aur humans mein)

The RNAi Mechanism: Step-by-Step Derivation

Chalte hain derive karte hain ki ek miRNA ek gene ko kaise silence karta hai, DNA blueprint se shuru karke.

Step 1: Transcription of pri-miRNA

Kya hota hai: Ek primary miRNA (pri-miRNA) transcript RNA Polymerase II dwara ek miRNA gene se banaya jata hai. Yeh ek lamba (~1000 nt) RNA hota hai hairpin structure ke saath.

Yeh step kyun? miRNA genes genome mein scattered hote hain (often doosre genes ke introns mein ya intergenic regions mein). Cell inhe initially regular genes ki tarah treat karta hai — transcription raw material create karta hai.

DNA (miRNA gene) RNA Pol II pri-miRNA (long hairpin)

Step 2: Processing to pre-miRNA (Nuclear Cropping)

Kya hota hai: Drosha naam ka ek enzyme (ek RNase III endonuclease) pri-miRNA hairpin ko stem ke base se ~11 nucleotides door kaatta hai.

Yeh step kyun? Poora pri-miRNA nucleus se export karne ke liye bahut bulky hota hai. Drosha ise ~70 nt ke precursor miRNA (pre-miRNA) mein trim karta hai jisme 2-nucleotide 3' overhang hota hai (export ke liye ek molecular "barcode").

pri-miRNA Drosha + DGCR8 pre-miRNA (stemloop, 70 nt)

Key detail: Drosha ek partner protein DGCR8 ke saath kaam karta hai jo double-stranded stem ko recognize karta hai.

Step 3: Export to Cytoplasm

Kya hota hai: Exportin-5 (ek transport protein) 3' overhang ko recognize karta hai aur pre-miRNA ko nuclear pores ke through cytoplasm mein shuttle karta hai.

Yeh step kyun? Protein synthesis machinery (ribosomes) cytoplasm mein hoti hai. miRNA ko apne mRNA targets tak pahunchna zaroori hai jahan translation hoti hai.

pre-miRNA (nucleus) Exportin-5 + GTP pre-miRNA (cytoplasm)

Step 4: Dicer Cleavage

Kya hota hai: Dicer (ek aur RNase III enzyme) pre-miRNA loop ko kaatta hai, ek ~22 bp double-stranded RNA duplex chhod ke.

Yeh step kyun? Dicer Drosha cut se exactly 22 nucleotides measure karta hai — yeh precision ensure karti hai ki sahi "seed region" preserve ho. Loop junk ki tarah discard ho jata hai.

pre-miRNA Dicer miRNA duplex (22 bp) + loop fragment

Derivation detail: Dicer ka ek PAZ domain hota hai (3' overhang se bind karta hai) aur do RNase III domains jo ~22 nt apart spaced hote hain. Yeh ek molecular ruler hai.

Step 5: RISC Loading and Strand Selection

Kya hota hai: miRNA duplex ko RISC (RNA-Induced Silencing Complex) par load kiya jata hai, ek protein complex jo Argonaute (AGO) proteins contain karta hai. Ek strand (guide strand) rakha jata hai; doosra (passenger strand) degrade ho jata hai.

Yeh step kyun? Target dhundhne ke liye sirf ek strand chahiye. Cell us strand ko guide select karta hai jiska 5' end kam stable hota hai (lower base-pairing energy).

miRNA duplex + AGO proteins \to RISC (mature miRNA loaded) + passenger strand (degraded)

Selection rule:

Δ G_{5^{'} -end, guide} < Δ G_{5^{'} -end, passenger} ⟹ guide selected

Step 6: Target Recognition and Silencing

Kya hota hai: RISC mein mature miRNA mRNAs ko complementary sequences ke liye scan karta hai (usually 3' UTR mein). Binding se do mein se ek outcome hota hai:

Perfect complementarity (animals mein rare): AGO mRNA ko scissors ki tarah cleave karta hai → degradation
Imperfect match (typical): RISC ribosome binding block karta hai ya deadenylases recruit karta hai → translation repression ya slow decay

RISC + target mRNA \to {mRNA cleavage translation block (perfect match) (imperfect match)

3' UTR kyun? Ribosome 5'→3' padhta hai, lekin 3' UTR translate karne ki zaroorat nahi. miRNAs wahan bind karte hain taaki coding sequence disrupt na ho lekin interfere phir bhi karein.

Seed region rule: miRNA ke nucleotides 2-8 zaroor target se base-pair karein. Positions 9-22 mein mismatches ho sakte hain ("wobbles").

Quantitative Impact: How Much Silencing?

Ek single miRNA typically target protein levels ko 30-80% reduce karta hai — complete knockout nahi. Math yeh hai:

Maano $P_{0}$ = miRNA ke bina protein synthesis rate, $P$ = miRNA ke saath rate, $k_{on}$ = RISC binding rate, $k_{off}$ = RISC unbinding rate.

Equilibrium par, mRNA ka fraction jo RISC se bound hai:

f_{bound} = \frac{[ RISC ] \cdot k _{on}}{[ RISC ] \cdot k _{on} + k _{off}}

Agar bound mRNA ki translation efficiency $ϵ$ hai (0 se 1 tak), to:

\frac{P}{P _{0}} = 1 - f_{bound} + ϵ \cdot f_{bound} = 1 - f_{bound} (1 - ϵ)

Example: Agar 70% mRNAs bound hain ( $f_{bound} = 0.7$ ) aur bound mRNAs 90% silenced hain ( $ϵ = 0.1$ ):

\frac{P}{P _{0}} = 1 - 0.7 (1 - 0.1) = 1 - 0.63 = 0.37 (63% reduction)

Worked example Worked Example 1: let-7 in Development

Scenario: C. elegans larvae mein, let-7 miRNA absent hota hai. Final larval stage ke baad, let-7 transcribe hota hai. Uska target lin-41 mRNA hai (ek protein code karta hai jo adult cell fates block karta hai). Step-by-step:

Larva: No let-7 → lin-41 mRNA translate hota hai → LIN-41 protein accumulate hota hai → cells larval program mein rehti hain
Adult transition: let-7 gene transcribe hota hai → pri-let-7 → Drosha cuts → pre-let-7 → Exportin-5 → Dicer cuts → let-7 duplex → RISC
Silencing: let-7-RISC lin-41 3' UTR mein saat sites se bind karta hai (imperfect matches, seed regions align karte hain) → ribosomes block → LIN-41 protein 80% drop → adult cell fates activate

Har step kyun?

Drosha/Dicer precision ensure karta hai ki seed region (GAGUAG) exactly positioned ho
Multiple binding sites silencing amplify karte hain (har RISC complex independent hai, multiplicative effect)
Timing: let-7 tabhi banta hai jab animal ek threshold size reach kare (hormone signal), isliye switch developmental hai

Worked example Worked Example 2: miR-122 in Hepatitis C

Scenario: Hepatitis C virus (HCV) RNA ke 5' UTR mein miR-122 ke liye do binding sites hain. Paradoxically, miR-122 HCV RNA ko silence karne ki jagah stabilize karta hai.

Step-by-step:

HCV liver cell mein enter karta hai → viral RNA release hota hai
Liver cells high miR-122 express karti hain (hepatocytes mein sabhi miRNAs ka 70%)
miR-122-RISC HCV 5' UTR se bind karta hai (ribosome entry site ke paas)
Translation block karne ki jagah: RISC binding 5' end ko Xrn1 exonuclease se degradation se protect karta hai
HCV RNA half-life 10-fold badhti hai → zyada viral proteins → zyada virions Yeh step kyun? Normally, cellular mRNAs mein ek 5' cap hota hai jo exonucleases block karta hai. HCV RNA mein cap nahi hota, isliye Xrn1 usse 5' end se chaba leta. miR-122-RISC ek "prosthetic cap" ki tarah act karta hai. Yeh RNAi machinery ka viral hijack hai.

Clinical relevance: miR-122 block karne wali drugs (jaise Miravirsen, antisense oligonucleotide) patients mein HCV RNA ~2 log₁₀ reduce karti hain.

Small Interfering RNAs (siRNAs): The Exogenous Cousins

Mechanism difference:

dsRNA (viral) Dicer only (no Drosha) siRNA duplex RISC (same AGO) perfect match \to mRNA cleavage

Cell ko kyun care hai: Viruses replication ke dauran dsRNA inject karte hain. RNAi pathway viral genomes chop karne ke liye evolve hua.

Biological Roles: Why Does RNAi Exist?

Fine-tuning gene expression: Zyaadatar miRNAs apne targets ko 2-5 fold reduce karte hain, ON/OFF nahi. Isse graded responses milte hain (e.g., miR-1 calcium channel mRNA adjust karke heart contraction strength tune karta hai).
Noise buffering: Agar ek transcription burst bahut zyada mRNA banata hai, miRNAs excess trim karte hain. Math: Agar mRNA noise $σ_{m}^{2}$ high hai, miRNA-mediated decay ek damping term add karta hai:
$\frac{d [ mRNA ]}{d t} = k_{transcription} - k_{decay} [mRNA] - k_{RNAi} [RISC] [mRNA]$
$k_{RNAi}$ term fluctuations ko flatten karta hai.
Developmental switches: let-7, lin-4, miR-125 ancient timers hain jo metamorphosis, differentiation, ya cell cycle exit trigger karte hain.
Antiviral defense: siRNAs viral RNA destroy karte hain. Plants aur invertebrates mein, yeh primary immune system hai. Mammals ne interferon evolve kiya, lekin RNAi abhi bhi exist karta hai.
Transposon silencing: Transposons (jumping genes) ko piRNAs (ek aur small RNA class) RNAi-like mechanisms ke zariye check mein rakhte hain.

Common mistake Common Error: "miRNAs Delete Genes"

Galat idea: "Agar ek miRNA kisi gene ko target karta hai, to woh gene knockout ki tarah band ho jata hai." Kyun sahi lagta hai: "Silencing" term absolute lagti hai. Cleaved mRNAs destroy ho jaati hain.

Fix: Zyaadatar animal miRNAs partial repression karte hain (30-80% reduction), elimination nahi. Target mRNA abhi bhi present aur translate hoti hai, bas kam. Dimmer switch socho, light switch nahi. Saath hi, miRNAs often multiple genes target karte hain (ek miRNA ke 100+ targets ho sakte hain), isliye effect distributed hota hai. Ek knockout mutant (e.g., let-7 deletion) dramatic phenotypes dikhata hai, lekin let-7 overexpress karne se har target sirf modestly reduce hota hai.

Proof: miR-200 overexpress karte cells ka RNA-seq dikhata hai ki target mRNAs normal ke 40-60% tak drop hoti hain, zero nahi. Ribosome profiling dikhata hai ki kuch translation abhi bhi hoti hai.

Recall Feynman: Explain to a 12-Year-Old

Socho tumhari cells ek factory ki tarah hain jo instruction manuals (mRNA) se toys (proteins) banati hai. Kabhi kabhi factory ek toy bahut zyada bana leti hai, jo materials waste karta hai. Toh factory mein chote robots hote hain jinhe miRNAs kehte hain. Har robot ke paas ek scanner hota hai jo kuch manuals par ek specific barcode (seed region) dhundhta hai.

Jab ek miRNA robot apna matching manual dhundh leta hai, toh woh uss par ek sticker lagata hai jis par likha hota hai "slow down" ya "ise throw away karo." Toy-making machines (ribosomes) sticker dekh ke ya to us manual ko skip kar deti hain ya slower kaam karti hain. Kuch ghanton mein, unwanted toys ki pile shrink ho jaati hai.

Yahan cool part hai: miRNA robots usi DNA cookbook se bane hote hain jaise baki sab kuch, lekin ek special tarike se. Pehle, RNA ka ek lamba tukda hairpin shape mein fold hota hai (bobby pin ki tarah). Drosha naam ka ek enzyme ise short karta hai, phir Dicer naam ka doosra enzyme ise aur short karta hai, ek tiny double-stranded piece banaate hue. Ek strand robot ka scanner ban jaata hai, aur woh doosre proteins ke saath milkar RISC team banaata hai.

Factory in robots ko quickly changes ke response mein use karti hai — agar environment kehta hai "yeh toy banana band karo," ek miRNA turn on karna DNA instructions rewrite karne se faster hai. Isliye miRNAs grow karne aur adapt karne ke liye itne important hain.

Seed region positions 2-8 hai → "28 days later, miRNA ne apna target dhundh liya" (numbering yaad rakhne ke liye zombie movie reference).

Connections

DNA Methylation and Histone Modifications: Chromatin state determine karta hai ki kaun se miRNA genes transcribe honge
Transcription Factors: Kaafi transcription factors pri-miRNA expression regulate karte hain (e.g., p53 miR-34 induce karta hai)
mRNA Splicing: Kuch miRNAs introns mein embedded hote hain aur splicing ke dauran release hote hain
Post-Translational Modifications: RNAi mRNA reduce karta hai, PTMs existing protein regulate karte hain — complementary strategies
Viral Replication: Kaafi viruses anti-RNAi proteins encode karte hain (e.g., influenza ka NS1 Dicer block karta hai)
Cancer Biology: Tumors often tumor-suppressor miRNAs downregulate karte hain (e.g., let-7 RAS oncogenes target karta hai)
Epigenetics: DNA methylation miRNA genes silence kar sakta hai → RNAi ka loss → disease

#flashcards/biology

miRNA process karne wale do key enzymes kaun se hain, aur har ek kahan act karta hai? :: Drosha (nucleus mein act karta hai, pri-miRNA ko pre-miRNA mein kaatta hai) aur Dicer (cytoplasm mein act karta hai, pre-miRNA ko mature miRNA duplex mein kaatta hai).

miRNA ka seed region kya hai, aur yeh kyun critical hai?

Seed region mature miRNA ke nucleotides 2-8 hote hain. Ise target mRNA ke 3' UTR ke saath perfectly base-pair karna zaroori hai taaki RISC bind kare aur target silence kare. Seed mein mismatches silencing khatam kar dete hain.

miRNA aur siRNA mein origin aur complementarity ke hisaab se kya fark hai?

miRNAs endogenous hote hain (genome mein encoded), hairpin precursors se processed, aur imperfect target complementarity rakhte hain (usually translation repress karte hain). siRNAs exogenous hote hain (dsRNA, viruses, ya lab se), perfect complementarity rakhte hain, aur mRNA cleavage trigger karte hain.

Mature miRNA konsa complex load karta hai aur gene silencing execute karta hai?

RISC (RNA-Induced Silencing Complex), jisme Argonaute (AGO) proteins catalytic component ki tarah hote hain. AGO guide strand se bind karta hai aur complementary mRNA targets scan karta hai.

Zyaadatar miRNAs mRNAs ke 3' UTR se kyun bind karte hain?

3' UTR translate nahi hota, isliye wahan miRNA binding coding sequence disrupt nahi karti. Phir bhi yeh ribosome binding region ke itna paas hota hai ki translation initiation mein interfere kar sake ya mRNA decay ke liye deadenylases recruit kar sake.

let-7 example mein, let-7 absent vs. present hone par kya hota hai?

Absent: lin-41 mRNA fully translate hoti hai → LIN-41 protein cell ko larval state mein rakhta hai. Present: let-7-RISC lin-41 3' UTR se bind karta hai → translation block → LIN-41 drop → adult differentiation aage badhti hai.

Drosha pri-miRNA ko cleavage ke liye kaise recognize karta hai?

Drosha DGCR8 ke saath partner karta hai, jo hairpin structure ke double-stranded stem ko recognize karta hai. Drosha phir stem ke base se ~11 nucleotides door kaatta hai, pre-miRNA par 2-nt 3' overhang chodta hai.

RISC mein miRNA duplex ka kaun sa strand guide strand banta hai yeh kya determine karta hai?

Woh strand jo less stable 5' end rakhta hai (lower base-pairing free energy) guide select hota hai. Doosra strand (passenger) degrade ho jaata hai.