Q.Ribosomes of eukaryotes are
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Prokaryotic Ribosomes and Inclusion Bodies
Imagine a factory floor where proteins are the products. The ribosome is the assembly machine that reads the blueprint (mRNA) and links amino acids together. In a prokaryotic cell, there is no separate "packaging room" (no membrane-bound organelles), so the machines and the raw material stores sit directly in the cytoplasm.
The Ribosome: The Protein Factory
A prokaryotic ribosome is a massive complex of RNA and protein, built from two subunits that clamp onto mRNA. The 70S designation comes from its sedimentation rate in a centrifuge — the "S" stands for Svedberg units, a measure of size and shape, not a simple sum of masses. The large subunit is 50S, the small is 30S; together they make 70S.
Prokaryotic ribosomes are 70S (50S + 30S subunits). Eukaryotic ribosomes are 80S (60S + 40S). This difference is why some antibiotics (like streptomycin) can target bacterial ribosomes without harming human cells.
Now, a single ribosome can translate one mRNA, but that is slow. In a prokaryotic cell, as soon as the 5' end of an mRNA emerges from transcription, ribosomes hop on. Multiple ribosomes translate the same mRNA simultaneously, each at a different point along the strand. This structure is called a polysome (polyribosome). It looks like a string of beads under an electron microscope — each bead is a ribosome, and the string is the mRNA.
Polysomes massively boost protein output. One mRNA can be read by 10–20 ribosomes at once, producing many copies of the same protein in the time it takes one ribosome to finish.
Inclusion Bodies: The Storage Rooms
Prokaryotes have no membrane-bound organelles like lysosomes or vacuoles. Instead, they store reserve materials directly in the cytoplasm as inclusion bodies. These are simply aggregates of molecules, often visible under a light microscope as granules. They are not surrounded by a membrane — they are just dense clumps of stuff.
The key point: inclusion bodies are non-membrane bound and serve as stockpiles for when nutrients run low.
Three common types you need to know:
| Inclusion Body | What It Stores | Why It Matters |
|---|---|---|
| Glycogen granules | Polymer of glucose | Energy reserve — broken down when carbon is scarce |
| Phosphate granules (volutin) | Inorganic polyphosphate | Phosphate and energy reserve — used for ATP synthesis later |
| Gas vacuoles | Air-filled protein shells | Buoyancy — lets cyanobacteria float to optimal light depth |
Eukaryotic ribosomes are 80S (composed of 60S and 40S subunits); 70S ribosomes (50S+30S) are found in prokaryotes and in eukaryotic orga …
[!TLDR]
(a) 80S
Why
Eukaryotic ribosomes are 80S (composed of 60S and 40S subunits); 70S ribosomes (50S+30S) are found in prokaryotes and in eukaryo …
- CBSE 2025Set ANNUAL1 markMCQQ.What is true about ribosomes?(a) The prokaryotic ribosomes are 80 S, where 'S' stands for sedimentation coefficient.(b) These are composed of ribonucleic acid and protein.(c) These are found only in eukaryotic cells.(d) These are self-splicing introns of some RNA's.
›Reveal solutionSolution
Ribosomes are ribonucleoprotein particles made of rRNA and protein, present in all cells (prokaryotic and eukaryotic), and are the site of protein synthesis.
Let's check each statement:
- (a) is incorrect: prokaryotic ribosomes are 70S (made of a 50S and a 30S subunit), not 80S. 80S is the eukaryotic ribosome size (60S + 40S subunits).
- (b) is correct: every ribosome, whether 70S or 80S, is composed of ribosomal RNA (rRNA) combined with a set of ribosomal proteins. …
- CBSE 2024Set ANNUAL1 markMCQQ.Ribosomes of eukaryotes are(a)(a) 80S(b)(b) 70S(c)(c) 50S(d)(d) 30S
›Reveal solutionSolution
[!TLDR]
(a) 80S
Why
Eukaryotic ribosomes are 80S (composed of 60S and 40S subunits); 70S ribosomes (50S+30S) are found in prokaryotes and in eukaryo …
- CBSE 2020Set ANNUAL1 markMCQQ.Prokaryotic cells have ribosomes of :(a) 50s(b) 30s(c) 70s(d) 80s
›Reveal solutionSolution
Prokaryotic ribosomes are 70S, made of a 50S large subunit and a 30S small subunit.
Ribosome size is measured by sedimentation coefficient (Svedberg units, S), which reflects size/density rather than adding arithmetically. Prokaryotes (bacteria) have smaller 70S ribosomes composed of a 50S and a 30S subunit, found free in the cytoplasm. Eukaryotic cytoplasmic ribosomes are larger, 80S (60S + 40S subunits) -- though eukaryotic organelles like mitochondria and chloroplasts retain 70S ribosomes, consiste …
- CBSE 2020Set ANNUAL1 markQ.In which type of cells are 70s ribosomes found?
›Reveal solutionSolution
70S ribosomes occur in prokaryotes and in mitochondria/chloroplasts of eukaryotic cells.
Ribosomes are classified by their sedimentation coefficient (S, Svedberg unit). Prokaryotic cells (bacteria, cyanobacteria) have 70S ribosomes, composed of a 50S large subunit and 30S small subunit. Interestingly, the organelles of endosymbiotic origin — mitochondria and chloroplasts — inside eukaryotic cells also contain their own 70S ribosomes (supporting the endo …
- CBSE 2019Set ANNUAL1 markQ.What is a polysome ?
›Reveal solutionSolution
A polysome is a single mRNA strand with multiple ribosomes attached along its length, each independently translating it into a protein — an efficient way to mass-produce a protein from one transcript.
During protein synthesis (translation), a ribosome attaches near the 5' end of an mRNA strand and moves along it, reading the codons and adding amino acids to a growing polypeptide chain. Because the mRNA is much longer than the space occupied by a single ribosome, several ribosomes can attach to and move along the SAME mRNA one after another, each at a different stage of translating the same message.
This string of ribosomes strung along one mRNA, viewed under an electron microscope, looks like beads on a string, and is called a polysome or polyribosome.
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