transfer RNA (tRNA)
/ TRANS-fer AR-en-AY /
An mRNA is written in one alphabet (the four RNA letters), but a protein is written in another (the twenty amino acids). Something must translate between the two — a sort of bilingual dictionary that, for each RNA word, knows which amino acid it means and can physically fetch it. Transfer RNA is that adaptor. It is a small RNA molecule that reads one three-letter codon on the mRNA and brings the matching amino acid.
Each tRNA is a short strand of RNA folded into a compact, roughly L-shaped clover, with two business ends. At one end sits a three-letter sequence called the anticodon, which base-pairs with a matching codon on the mRNA. At the other end, the tRNA carries one specific amino acid, loaded on by a dedicated enzyme. So when a tRNA's anticodon locks onto a codon inside the ribosome, the correct amino acid is delivered to exactly the right spot in the growing protein chain.
tRNA is the linchpin that makes the genetic code physical rather than abstract. The code is essentially a chart pairing codons with amino acids, and the set of tRNAs (plus the enzymes that charge them) is the cell's hardware implementation of that chart. There are different tRNAs for different codons; clever pairing rules let a smaller number of tRNAs cover all sixty-one amino-acid codons. Without tRNA, an mRNA would just be an unread string of letters.
Think of a tRNA as a delivery worker holding an address label (the anticodon) in one hand and a specific package (the amino acid) in the other. When its label matches the mRNA's address, it drops its package into the growing protein and steps away to be reloaded.
A tRNA matches its anticodon to an mRNA codon and delivers the right amino acid.
tRNA is never translated into protein — it works as an RNA. Its job is reading codons and carrying amino acids, which is why it is one of the noncoding RNAs.