Tiny particle, the site of protein synthesis, that is present in large numbers in living cells. They occur both as free particles within cells and, in eukaryotes, as particles attached to the membranes of the endoplasmic reticulum. They are 40percnt protein and 60percnt RNA. Ribosomes account for a large proportion of the total RNA of a cell. Proteins newly formed on ribosomes detach and migrate to other parts of the cell to be used.

Learn more about ribosome with a free trial on Britannica.com.