Describe ribosome structure and role
2.3.5· Biology › Organelles & Their Functions
Ribosome kya hota hai?
Yeh kis cheez se bana hai: do subunits — ek large subunit aur ek small subunit — dono rRNA + proteins ka ek ghana jaal hain. Inhe nucleolus mein assemble kiya jata hai.
Membrane kyun nahi hoti? Kyunki iska kaam mechanical reading hai, compartmentalisation nahi. Agar membrane hoti to mRNA usse thread hone mein mushkil hoti. (Iska matlab yeh bhi hai ki antibiotics ise directly attack kar sakti hain.)
Sizes — Svedberg (S) units mein measure kiya jata hai
| Cell type | Whole ribosome | Large subunit | Small subunit |
|---|---|---|---|
| Prokaryotic (& mitochondria/chloroplast) | 70S | 50S | 30S |
| Eukaryotic (cytoplasm) | 80S | 60S | 40S |
Structure — working parts
rRNA catalysis kyun karta hai (protein kyun nahi): Experiments mein protein hata dene ke baad bhi catalytic activity bachi rehti hai — rRNA khud active site banata hai. Yeh RNA-world hypothesis ka bada evidence hai (RNA info store bhi kar sakti hai aur catalysis bhi).
Kaam: Translation (step-by-step derive kiya gaya)
Hum logic ko first principles se build karte hain, memorise karne ki jagah.
Problem: mRNA codons ki ek string hai (har ek 3 bases). Humein har codon ko sahi amino acid mein convert karna hai, order mein.
Step 1 — Initiation. Kyun? Ek defined start chahiye. Small subunit mRNA se bind karta hai aur slide karke start codon AUG par aa jata hai. Met carry karne wala ek tRNA P site mein enter karta hai; large subunit clamp ho jata hai. P site pehle kyun? Taaki agli (A) site aane wale amino acid ke liye free rahe.
Step 2 — Elongation.
- Ek charged tRNA jiska anticodon A-site codon se match karta hai, bind karta hai (complementary base pairing). Pairing kyun? Yeh guarantee karta hai ki sahi amino acid code ke zariye select ho, randomly nahi.
- Peptidyl transferase P-site chain aur A-site amino acid ke beech ek peptide bond banata hai. Yeh direction kyun? Growing chain ko aage naye amino acid par handover kiya jata hai.
- Ribosome ek codon aage translocate karta hai: A→P, P→E, E→out. Move kyun karta hai? Taaki agla codon khaali A site mein aa jaye.
Step 3 — Termination. Ek stop codon (UAA/UAG/UGA) ka koi matching tRNA nahi hota → ek release factor enter karta hai → polypeptide free ho jata hai.
Free vs Bound Ribosomes
Worked Examples
Flashcards
What two molecules make up a ribosome?
Where are ribosomes assembled in eukaryotes?
What size are prokaryotic vs eukaryotic ribosomes?
What are the subunit sizes of a 70S ribosome?
What are the subunit sizes of an 80S ribosome?
Why doesn't 50S + 30S = 80S?
Name the three tRNA sites on a ribosome.
What enzyme forms peptide bonds, and what is it made of?
What process do ribosomes carry out?
What is the start codon?
Name the three stop codons.
Free vs bound ribosomes — what do their proteins do?
For a coding region of n bases, how many amino acids?
Why do mitochondria have 70S ribosomes?
Recall Feynman: explain to a 12-year-old
Sochho ek sentence ek secret code mein likhi hai jahan har 3 letters = 1 LEGO brick. Ribosome ek chhoti reading-machine hai jo sentence ke saath-saath chalti hai, ek baar mein 3 letters padhti hai, matching LEGO brick uthati hai, aur chain pe snap karti hai. Jab yeh "STOP" word par pahunchti hai, toh chhod deti hai aur tumhare paas ek finished LEGO toy hota hai — woh toy protein hai. Iske koi walls nahi hote (koi membrane nahi) kyunki ise sirf padhna aur banana hai, kuch store nahi karna.
Connections
- Translation — woh poora process jo ribosomes drive karte hain
- mRNA and the Genetic Code — codons jo ribosome padhta hai
- tRNA Structure and Function — amino acids deliver karta hai
- Rough Endoplasmic Reticulum — bound ribosomes ka ghar
- Nucleolus — jahan ribosomes assemble hote hain
- Endosymbiotic Theory — kyun mitochondria/chloroplasts mein 70S ribosomes hote hain
- Antibiotics and Selective Toxicity — 70S ko target karne wali drugs
- RNA World Hypothesis — ribozymes as evidence