Ribosome
A ribosome is the small structure in all cells that reads messenger RNA and assembles amino acids into proteins.
Definition
A ribosome is a tiny machine inside a cell that builds proteins by reading a copy of the instructions from DNA and linking amino acids together in the right order. It is a non-membrane-bound organelle made of ribosomal RNA and protein that reads the codons of a messenger RNA and joins matching amino acids delivered by transfer RNA into a polypeptide chain, either floating free in the cytoplasm or attached to the rough endoplasmic reticulum. Structurally it is a ribonucleoprotein complex with a small subunit that decodes mRNA and a large subunit that catalyzes peptide bond formation: 30S and 50S subunits (70S assembled) in bacteria, 40S and 60S (80S) in eukaryotic cytoplasm. The peptidyl transferase center is made of ribosomal RNA, making the ribosome a ribozyme. Translation proceeds through initiation, elongation (aminoacyl-tRNA delivery, peptide bond formation, and translocation, each driven by GTP hydrolysis), and termination at a stop codon.
Example
The protein in your hair, the enzymes that digest your lunch, and the antibodies that fight a cold were all assembled by ribosomes. Each codon is $3$ nucleotides long, so an mRNA with $900$ coding nucleotides produces a protein of $300$ amino acids, and since a bacterial ribosome adds about $15$ to $20$ amino acids per second, that protein takes roughly $15$ to $20$ seconds to build. A rapidly growing E. coli cell contains about $20{,}000$ ribosomes, roughly a quarter of its dry mass, and antibiotics such as tetracycline, erythromycin, and streptomycin work by binding bacterial ribosomes at sites that differ from eukaryotic ribosomes.
Key Insight
Ribosomes are found in every living cell, from bacteria to humans, one clue that all life shares a common ancestor. The instructions in DNA are useless until a ribosome turns them into proteins, and proteins do almost everything in a cell, so ribosomes are where genetic information becomes action. Because the catalytic core of the ribosome is RNA rather than protein, ribosomes are strong evidence for an early RNA world in which RNA both stored information and catalyzed reactions, and ribosomal RNA sequences remain the standard molecule for building the tree of life.