In prokaryotes, ribosomes are associated with the plasma membrane. They are about 15 nm by 20 nm in size (nanometre = 10-9 m).
These are granular structures composed of ribonucleic acid (RNA) and proteins, and not surrounded by any membrane.
Made of two subunits
- 50S unit
- 30S unit
But it adds up to 70S. Here, S is the Svedberg's unit and stands for the sedimentation coefficient which is indirectly a measure of density and size.
The Eukaryotic ribosomes are similar but are 80S and have subunits 60S and 40S. But chloroplasts and mitochondria in eukaryotes have 70S ribosomes.
George Palade first observed ribsomes under electron microscope as dense particles in 1953
Ribosomes are the site of protein synthesis.
Sometimes, several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. These ribosomes translate the mRNA into proteins.
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