Ribosome

The machine that reads the message and builds the protein, assembled from two subunits made in the nucleolus.

Two subunits, one machine

The nucleolus synthesises and processes precursor rRNA into mature rRNAs and assembles the 40S and 60S subunits, which later join as the 80S ribosome.

eukaryotic, review · review · Ponti D 2025, Int J Mol Sci · source

How fast it reads

13 to 18 amino acids per second, measured on single messages in living cells.

human cells · SunTag labelling of nascent protein, single-molecule imaging · Pichon X et al. 2016, J Cell Biol · source

How many on one message

About 1.3 ribosomes per kilobase of message, so a typical mRNA carries several at once as a polysome.

human cells · single-molecule imaging of endogenous and reporter mRNA · Pichon X et al. 2016, J Cell Biol · source

How many in a tissue

About 2 x 10^13 ribosomes per ml of liver, carried on the rough ER, which is two thirds of the 11 m2 of ER membrane in that volume.

rat liver, in situ · stereology on electron micrographs · Weibel ER et al. 1969, J Cell Biol · source

Structure known to the atom

The first X-ray structure of a eukaryotic ribosome was solved at 3.0 angstrom resolution from yeast, following the bacterial structures before it.

yeast, Saccharomyces cerevisiae · X-ray crystallography · Jenner L et al. 2012, Curr Opin Struct Biol · source

Not shown yet

Diameter

No sourced value for this yet: none of the abstracts read for this page states a ribosome diameter, so the copy's size is a drawing choice and not a measurement.

Number per human cell

No sourced value for this yet: the count above is per millilitre of rat liver tissue, which is not a per-cell figure for any human cell, and converting it would need a cell count this page does not have.

Behind the copy

The ribosome is the machine that reads mRNA and builds protein. Its two subunits, 40S and 60S, are assembled separately in the nucleolus and join on a message as the 80S ribosome. In living human cells translation runs at 13 to 18 amino acids per second, with about 1.3 ribosomes per kilobase of message, so one mRNA usually carries several at once.

source 1, source 2

Other organelles

All · Autophagosome · Centrosome · Cilia and flagella · Cytoskeleton · Endosomes and vesicles · Golgi · Lipid droplet · Lysosome · Mitochondrion · Nuclear envelope · Nucleolus · Nucleus · Peroxisome · Plasma membrane · Proteasome · Rough ER · Smooth endoplasmic reticulum

Building the copy.

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Stain

No stain on cyto.site's table names a part this copy draws.

Real pictures

Panel of a six-part figure, cropped to this panel and otherwise unaltered, which CC BY allows with attribution. Human astrocyte differentiated from induced pluripotent stem cells of a healthy donor, 25 days in vitro. The authors describe the protein-synthesising apparatus here as polysomes, marked by the yellow arrow, with several short cisternae of rough ER at the red arrow. At a 0.5 micrometre scale bar the fine dark granules filling the cytoplasm are the ribosomes themselves: this is about as close as conventional electron microscopy brings a reader to one. Kutukova, Ivanov, Novosadova et al 2026, Modern Technologies in Medicine, CC BY 4.0 · source · transmission electron micrograph, panel f of figure 2

The authors' caption for the figure reads: "Ribosomes colocalize with the SR Representative electron micrographs of adult rat ventricular myocytes show (A) ribosome (arrowheads) association with perinuclear membranes (ER), (B) peripheral membranes (longitudinal, free SR), (C) T‐tubules (near junctional SR) and (D) in the subsarcolemmal region, specifically at the terminal sarcolemma (TS) of intercalated disc (ID). M, mitochondria; T, T‐tubules; N, nucleus." Cropped to panels A to D, the electron micrographs; panels E and F, mouse immunofluorescence, are not shown. Sandmann, Brandenburg, Uhlenkamp et al 2026, The Journal of Physiology. Cropped by cyto.site to panels A to D, CC BY 4.0 · source · transmission electron microscopy