What is Transfer RNA and its synthetases?

A transfer RNA (Chapter 11) is the adaptor between codon and amino acid: its anticodon, three bases in a loop, pairs antiparallel with a codon of the message, and its $3'$ end carries the corresponding amino acid. The pairing at the third codon position is loose (wobble), so about forty tRNAs suffice for sixty-one codons. Each tRNA is loaded by its own aminoacyl-tRNA synthetase, an enzyme that recognises both the amino acid and the tRNA and joins them in two steps at the cost of one ATP (split to AMP: two high-energy bonds), proofreading the amino acid as it does so. These twenty enzymes are where the code is actually implemented: the ribosome does not check which amino acid a tRNA carries, only that its anticodon matches.