Ribosomes are the site of protein synthesis.They are either free in the cytoplasm or attached to the rough endoplasmic reticulum in eukaryotes.They are made in the nucleolus from ribosomal RNA (rRNA) and protein.Each ribosome is made from a large subunit and a small subunit. The large subunit contains the enzyme peptidyl transferase which joins amino-acids together to form proteins.Mitochondria and chloroplasts also contain ribosomes.The mitochondrial ribosomes of eukaryotic cells are distinct from their other ribosomes. They functionally resemble those in bacteria, giving rise to the evolutionary theory of the origin of mitochondria as endosymbiotic bacteria.