Describe ribosome structure and role
What is a ribosome?
WHAT it is made of: two subunits — a large subunit and a small subunit — each a tangle of rRNA + proteins. They are made (assembled) in the nucleolus.
WHY no membrane? Because its job is mechanical reading, not compartmentalisation. A membrane would only get in the way of mRNA threading through it. (This also means antibiotics can attack it directly.)
Sizes — measured in Svedberg (S) units
| Cell type | Whole ribosome | Large subunit | Small subunit |
|---|---|---|---|
| Prokaryotic (& mitochondria/chloroplast) | 70S | 50S | 30S |
| Eukaryotic (cytoplasm) | 80S | 60S | 40S |
Structure — the working parts
WHY rRNA does the catalysis (not protein): Experiments stripping away protein leave catalytic activity intact — the rRNA itself forms the active site. This is huge evidence for the RNA-world hypothesis (RNA can both store info and catalyse).
The role: Translation (derived step-by-step)
We build the logic from first principles instead of memorising.
Problem: mRNA is a string of codons (3 bases each). We must convert each codon into the correct amino acid, in order.
Step 1 — Initiation. Why? You need a defined start. The small subunit binds mRNA and slides to the start codon AUG. A tRNA carrying Met enters the P site; the large subunit clamps on. Why P site first? So the next (A) site is free for the incoming amino acid.
Step 2 — Elongation.
- A charged tRNA whose anticodon matches the A-site codon binds (complementary base pairing). Why pairing? It guarantees the right amino acid is selected by the code, not at random.
- Peptidyl transferase forms a peptide bond between the P-site chain and the A-site amino acid. Why this direction? The growing chain is handed forward onto the new amino acid.
- The ribosome translocates one codon along: A→P, P→E, E→out. Why move? So the next codon sits in the empty A site.
Step 3 — Termination. A stop codon (UAA/UAG/UGA) has no matching tRNA → a release factor enters → the polypeptide is freed.
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
Imagine a sentence written in a secret code where every 3 letters = 1 LEGO brick. The ribosome is a little reading-machine that runs along the sentence, reads 3 letters at a time, grabs the matching LEGO brick, and snaps it onto the chain. When it reaches the word "STOP", it lets go and you have a finished LEGO toy — that toy is the protein. It has no walls (no membrane) because it just needs to read and build, not store anything.
Connections
- Translation — the full process ribosomes drive
- mRNA and the Genetic Code — codons the ribosome reads
- tRNA Structure and Function — delivers amino acids
- Rough Endoplasmic Reticulum — home of bound ribosomes
- Nucleolus — where ribosomes are assembled
- Endosymbiotic Theory — why mitochondria/chloroplasts have 70S ribosomes
- Antibiotics and Selective Toxicity — drugs targeting 70S
- RNA World Hypothesis — ribozymes as evidence
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, ribosome basically cell ki protein factory hai. Gene ya mRNA sirf instruction hote hain — likhi hui recipe. Lekin recipe se khud-ba-khud khaana nahi banta; koi machine chahiye jo recipe padhe aur dish banaye. Wahi kaam ribosome karta hai — woh mRNA ko teen-teen base (codon) karke padhta hai aur correct amino acid jod-jodke protein chain banata hai. Isiliye ribosome ko translation ka machine kehte hain.
Structure mein do parts hote hain — large subunit aur small subunit, dono rRNA + protein se bane. Small subunit mRNA pakadta hai, large subunit mein A, P, E teen sites hote hain jahan tRNA aate-jaate hain (yaad rakho: APE leaves). Peptide bond banane wala enzyme peptidyl transferase actually rRNA hi hai — matlab ribosome ek ribozyme hai. Aur dhyan do: prokaryotes mein 70S, eukaryotes ke cytoplasm mein 80S. Yeh "S" Svedberg unit hai jo sinking speed batati hai, isliye 50S+30S = 70S — add nahi hota, ghabrao mat, yeh normal hai.
Kyun important hai? Kyunki bina protein ke koi enzyme, structure, ya antibody nahi banega — life ruk jaayegi. Aur ek smart point: antibiotics bacteria ke 70S ribosome ko target karte hain par humare 80S ko nahi — isiliye woh humein nuksan kiye bina bacteria maar dete hain. (Halki si exception: humare mitochondria mein bhi 70S hota hai kyunki woh purane bacteria se aaye the — endosymbiosis!) Exam mein "amino acids = bases/3" aur "peptide bonds = amino acids − 1" formula zaroor kaam aayega.