Translation

mRNA is written in a four-letter nucleotide alphabet, but proteins are built from twenty different amino acids. Nothing about a codon's shape lets it grab an amino acid directly, so the cell needs a translator. That's tRNA: one end has an anticodon that base-pairs with a complementary mRNA codon, the other end carries the exact amino acid that codon calls for. The ribosome is the workbench that holds mRNA and tRNA in place so this matching happens in order, one codon at a time.

Translation starts when the ribosome assembles around the start codon, AUG, which also sets the reading frame and codes for methionine. During elongation, a tRNA with a matching anticodon slides into the ribosome's active site, the ribosome catalyzes a peptide bond between the new amino acid and the growing chain, and the ribosome then shifts exactly one codon down the mRNA, ejecting the spent tRNA and opening the next codon for reading. This repeats until a stop codon, UAA, UAG, or UGA, appears. No tRNA has an anticodon for these, so instead a release factor protein fits into the ribosome, recognizes the stop codon directly, and triggers release of the finished polypeptide with no amino acid added.

Translation is the step that actually builds the product transcription only wrote instructions for, which is why anything that controls translation rate controls how much protein a cell actually makes, not just how much mRNA it has. In bacteria, which have no nucleus, ribosomes can latch onto an mRNA and start translating its front end while RNA polymerase is still transcribing its back end, coupling the two processes in real time.