The ribosome is the central component of the protein synthesis machinery in the cell. It contains both RNA and protein and is composed of two subunits. Ribosomes are essential for accurate translation of the mRNA-encoded genetic message; they biosynthesize proteins by catalyzing the peptide bond formation between tRNA-bound amino acids.
Ribosomes are composed of about 65 % RNA and 35 % proteins distributed into a small and a large subunit. In contrast to the ribosomal proteins (r-proteins), the ribosomal RNA (rRNA) has been highly conserved during evolution and contains numerous posttranscriptional modifications and peculiar structural features like pseudo-knots and other RNA-RNA tertiary interactions. Ribosomes are present in all organisms from the three domains of life (Bacteria, Archaea, and Eukarya) and in two eukaryotic organelles originated by ancient endosymbiosis of bacteria (mitochondrion and chloroplast).