Other meanings of Ribosome
Cell biology
The ribosome is a complex molecular machine found in all living cells that serves as the site of protein synthesis, or translation. It consists of two subunits—one large and one small—composed of ribosomal RNA and ribosomal proteins. Ribosomes read the sequence of messenger RNA (mRNA) and catalyze the formation of peptide bonds between amino acids carried by transfer RNA (tRNA).
The ribosome is composed of two unequally sized subunits, each containing ribosomal RNA (rRNA) and ribosomal proteins. In prokaryotes, the small subunit (30S) contains the 16S rRNA, while the large subunit (50S) contains 23S and 5S rRNA; together they form a 70S ribosome.1 Eukaryotic ribosomes are larger (80S) with a 40S small subunit (18S rRNA) and a 60S large subunit (28S, 5.8S, and 5S rRNA).2 The ribosome possesses three tRNA-binding sites: the A (aminoacyl) site, P (peptidyl) site, and E (exit) site. The catalytic activity of the ribosome resides in the rRNA, making it a ribozyme.3 Cryo-electron microscopy has resolved the atomic structure of both prokaryotic and eukaryotic ribosomes, revealing the intricate RNA–protein interactions that stabilize the complex.
Protein synthesis begins when the small subunit binds to mRNA and the initiator tRNA, followed by assembly of the large subunit. The ribosome then moves along the mRNA in a process called elongation, reading the genetic code in triplets (codons).4 Each incoming aminoacyl-tRNA enters the A site; peptide bond formation is catalyzed by the peptidyl transferase center of the large subunit. The ribosome then translocates, shifting the tRNAs from A to P to E sites, requiring GTP hydrolysis by elongation factors. Termination occurs when a stop codon is reached, recognized by release factors that trigger the release of the polypeptide chain.2 Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (polysome).
Ribosome biogenesis is tightly regulated and linked to cell growth and proliferation. In eukaryotes, rRNA is transcribed by RNA polymerase I in the nucleolus and assembled into ribosomal subunits before export to the cytoplasm.5 Defects in ribosome assembly or function can lead to diseases called ribosomopathies, such as Diamond-Blackfan anemia and Shwachman-Diamond syndrome. Many antibiotics target bacterial ribosomes without affecting eukaryotic ones, including tetracycline (blocking tRNA binding), chloramphenicol (inhibiting peptidyl transferase), and macrolides (blocking the exit tunnel).1 Some viruses, including SARS-CoV-2, hijack host ribosomes to translate their own proteins, and certain toxins (e.g., ricin) inactivate ribosomes by depurinating rRNA.
Though ribosomes are primarily known for translation, they also participate in co-translational protein folding, with the nascent chain emerging from the exit tunnel and interacting with chaperones. The discovery of the ribosome is credited to George Palade, who used electron microscopy in the 1950s and won a Nobel Prize in 1974.1 Ribosomes are roughly two-thirds RNA and one-third protein by mass, underscoring the catalytic role of RNA. Mitochondria and chloroplasts contain their own 70S ribosomes, similar to bacteria, supporting the endosymbiotic theory.2 Archaeal ribosomes are closer to those of bacteria in size but share some eukaryotic-like features. The ribosome is a major target for antibiotics, and resistance mutations in ribosomal RNA or proteins are a growing clinical problem. Additionally, the ribosome can stall during translation, triggering quality-control pathways to resolve stalled complexes.
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