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Episode 4 - Ribosomes and Translation

The ribosome is present in all types of cells. Unlike most organelles, it is not membrane-bound: it consists of two ribosomal sub-units, made up of ribosomal RNA and protein. These ribosomal subunits are made in the nucleolus and then exported out into the cytoplasm to “look for” mRNA that need translating. The full structure of a ribosome is only assembled when the ribosome is functionally active. In other words, a ribosome is always hard at work!

Noller, 1997

The ribosome functions in translation: to translate mRNA strands into polypeptide chains. Cells functioning to produce proteins (like plasma cells and pancreatic cells) have millions of ribosomes. Free-floating ribosomes are suspended in the cytosol and produce polypeptides/proteins that function within the cytosol (like the enzymes that catalyze the first few stages of glycolysis). Membrane-bound ribosomes are the starting point of the endomembrane system, generally producing proteins that are destined to become membrane-proteins in other organelles or for secretion outside the cell.

Ribosomes are classified by size according to their Sedimentation Coefficients (the rate of sedimentation in an ultracentrifuge), measured using Svedberg Units (S).

Prokaryotic cells have 70S ribosomes with a 50S large ribosomal subunit and a 30S small ribosomal subunit.
Eukaryotic cells have 80S ribosomes with a 60S large ribosomal subunit and a 40S small ribosomal subunit.

Ribosomes can form aggregates with other ribosomes into a Polyribosome, or a Polysome.

Polyribosomes enable many ribosomes to translate one mRNA transcript simultaneously, thus generating multiple copies of the same polypeptide chain in very quickly.
Polysomes are attached to membranes (of either the outer nuclear or the outer Rough ER) in eukaryotes and are free-floating in prokaryotes (as are all other ribosomes).

Campbell 1996

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