A ribosome is a macromolecular complex composed of ribosomal RNA and proteins that reads messenger RNA and catalyzes the assembly of amino acids into polypeptide chains.
Structure and Function
Ribosomes are the cellular machines that translate genetic information into proteins. Each ribosome consists of a small and a large subunit. In bacteria and archaea, the 70S ribosome comprises a 30S small subunit containing 16S rRNA and proteins, and a 50S large subunit containing 23S and 5S rRNA. Eukaryotic cytoplasmic ribosomes are larger (80S) and consist of a 40S subunit with 18S rRNA and a 60S subunit with 28S, 5.8S and 5S rRNA. The subunits assemble on messenger RNA during translation initiation. The small subunit decodes the mRNA codon sequence, while the large subunit catalyzes peptide bond formation between amino acids. Transfer RNAs carrying amino acids bind to three sites on the assembled ribosome—the aminoacyl (A) site, peptidyl (P) site and exit (E) site. The ribosome moves along the mRNA in a 5′ to 3′ direction, synthesizing the polypeptide from the N‑terminus to the C‑terminus. Once translation is complete, the subunits dissociate and can be reused.
Structural and Functional Diversity
Although ribosomes have a conserved core, their size and composition vary across the domains of life. Mitochondria and chloroplasts contain their own 55S‑70S ribosomes, reflecting their bacterial origin. Ribosomal RNA sequences, particularly 16S and 18S rRNA, are widely used for phylogenetic analyses and microbial identification. Many antibiotics selectively target bacterial ribosomes, exploiting differences between prokaryotic and eukaryotic ribosomes. Aminoglycosides interfere with decoding on the 30S subunit; macrolides and lincosamides block the peptide exit tunnel of the 50S subunit; and tetracyclines prevent aminoacyl‑tRNA binding. Because human mitochondrial ribosomes resemble bacterial ribosomes, some antibiotics affect mitochondrial protein synthesis, causing side effects. Ribosome biogenesis is a complex process requiring nucleolar assembly in eukaryotes and coordinated synthesis of rRNA and ribosomal proteins.
Ribosomes are central to gene expression, linking the information encoded in nucleic acids to the functional proteins that carry out cellular processes.
Related Terms: Protein synthesis, mRNA, tRNA, Translation, 70S ribosome, 80S ribosome