Ribosomes are the sites in a cell in which protein synthesis takes place. Cells have many ribosomes, and the exact number depends on how active a particular cell is in synthesizing proteins.

Within the ribosome, the mRNA and aminoacyl-tRNA complexes are held together closely, which facilitates base-pairing. The rRNA catalyzes the attachment of each new amino acid to the growing chain.

To build proteins, the cell makes a copy of DNA, called mRNA. Then, another molecule called a ribosome reads the mRNA, translating it into protein. But this step has been a visual mystery ...