Ribosome

A ribosome is a cellular structure composed of RNA and proteins that serves as the site of protein synthesis. Ribosomes -read- the sequence of a messenger RNA (mRNA) and translate it into a polypeptide chain (protein) by linking amino acids in the order specified by the mRNA.

• Ribosomes consist of two subunits (one large, one small) that come together during translation. In eukaryotes, cytoplasmic ribosomes are 80S (made of 60S + 40S subunits); in prokaryotes, ribosomes are smaller 70S (50S + 30S subunits).
• Location: Ribosomes may be free in the cytoplasm (making proteins for use in the cytosol) or bound to the rough ER (making proteins for secretion or membranes). Mitochondria and chloroplasts also have their own ribosomes similar to bacterial ones.
• Mechanism: Think of a ribosome as a molecular machine - it matches tRNAs to mRNA codons and catalyzes peptide bond formation, growing the polypeptide chain one amino acid at a time.

• Translation questions - If asked -what cellular machine assembles amino acids into proteins-- the answer is the ribosome. They might mention rRNA as a component since ribosomes are ribonucleoproteins.
• Antibiotic mechanism - Many antibiotics (e.g., tetracycline, erythromycin) target bacterial 70S ribosomes and not eukaryotic 80S ribosomes, exploiting this size/structure difference to inhibit bacterial protein synthesis without harming human cytoplasmic ribosomes.
• Sedimentation values - Exams sometimes expect recognition that eukaryotic ribosomes are 80S vs prokaryotic 70S. Remember these S values (Svedberg units) are a measure of size/density (and are not additive).