Biology · Molecular Basis of Inheritance · NEET
Both do the same job (making protein) but sit in different cells. The 70S ribosome is found in prokaryotes (like E. coli), and also inside mitochondria and chloroplasts. The 80S ribosome is found in the cytoplasm of eukaryotes. 70S is smaller and lighter than 80S. This exact pairing (70S = prokaryote, 80S = eukaryote) was directly tested in NEET 2025, so learn it firmly.
Because 'S' stands for Svedberg unit. It measures how fast a particle settles down in a centrifuge, which depends on both size and shape, not just weight. So the numbers do NOT add up like normal maths. When the 50S and 30S subunits join, the whole ribosome is 70S, not 80S. In the same way, 60S + 40S joins to make an 80S ribosome, not 100S. This is a very common NEET trap.
The 70S ribosome (prokaryote) has a large 50S subunit and a small 30S subunit. The 80S ribosome (eukaryote) has a large 60S subunit and a small 40S subunit. Remember the small subunit is the one that first meets the mRNA to start translation (NEET 2022 tested this).
A polysome, also called a polyribosome, is a group of many ribosomes attached to one single mRNA at the same time. Each ribosome makes its own copy of the protein, so the cell can make many protein copies from one mRNA very fast. NEET has asked this in 2016 and 2018 — the answer is always 'polysome'.
Prokaryotes such as E. coli have only 70S ribosomes. Their DNA is double-stranded, circular and naked (no histones, no nuclear membrane). Note: even inside eukaryotic cells, the mitochondria and chloroplasts contain 70S ribosomes — this supports the endosymbiotic theory. NEET 2019 (Odisha) tested the E. coli–70S link.
A ribosome is made of structural rRNAs and about 80 different proteins (NCERT Class 12, Ch 5). NEET 2023 asked this exact number and the answer is 80. Do not confuse this '80 proteins' with the '80S' size value — they are different things that just share the number 80.
From the statements given below choose the correct option: A. The eukaryotic ribosomes are 80S and prokaryotic ribosomes are 70S. B. Each ribosome has two sub-units. C. The two sub-units of 80S ribosome are 60S and 40S while that of 70S are 50S and 30S. D. The two sub-units of 80S ribosome are 60S and 20S and that of 70S are 50S and 20S. E. The two sub-units of 80S are 60S and 30S and that of 70S are 50S and 30S.
How many different proteins does the ribosome consist of?
Many ribosomes may associate with a single mRNA to form multiple copies of a polypeptide simultaneously. Such strings of ribosomes are termed as
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The 80S ribosome is bigger and heavier than the 70S ribosome. 80S is found in the eukaryotic cytoplasm; 70S is found in prokaryotes and inside mitochondria and chloroplasts.
Mitochondria and chloroplasts have 70S ribosomes, the same as prokaryotes. This is one piece of evidence for the endosymbiotic theory (they were once free-living bacteria).
S is the Svedberg unit. It measures how fast a particle sediments (settles) in a centrifuge, based on its size and shape. Because it is not simple mass, the subunit S values do not add up in a straight line (50S + 30S = 70S).
The small subunit binds the mRNA first. Translation begins the moment the small subunit meets the mRNA (NEET 2022). The two subunits join together only after this.
A ribosome is made of ribosomal RNA (rRNA) and about 80 different proteins. The rRNA of the large subunit also acts as a ribozyme (an RNA enzyme) that forms the peptide bond.
