Ribosomes

Within cells are small, dense structures known as ribosomes that catalyse the assembly of protein chains. The ribosome accomplishes the reading of the messenger RNAs and the binding of amino acids to the transfer RNAs to build up the protein structures.

There are free ribosomes that are suspended in the cytoplasm of the cell, but many of them are attached to the rough endoplasmic reticulum associated with the nuclear envelope of the cell.