The protein-building structures contained in all cells are ribosomes. Thousands of these structures are present in cells, with more present in cells that make lots of proteins. Ribosomes take the genetic message of DNA and translate it into proteins.
Sixty percent of a ribosome is ribosomal RNA, or rRNA. The other 40 percent is made of protein. Ribosomes are made in the nucleus, namely, the nucleolus, of a cell. A large and small subunit make up a ribosome. When not making proteins, the subunits exist separately in the cytoplasm. The two subunits join when proteins need to be made.
Ribosomes can exist independently in the cytoplasm of cells or be stuck to the membrane of the nucleus or the endoplasmic reticulum to form the rough endoplasmic reticulum. Unattached ribosomes generally make proteins for the cell's use, whereas those attached to the endoplasmic reticulum are usually bound for transport outside the cell.