Transfert RNAs (tRNA) act as adapter between nucleotides codons and amino acids. They pick up free amino acids in cytoplasm and carry them into the ribosomes where polypeptide chain is elongated.

tRNAs are polynucleotide of about 60 - 95 nucleotides long, including few specific nucleotids (dihydro-uridine, pseudo-uridine).
They exhibit a cloverleaf-like secondary structure consisting of a stem and three main loops. They also display a tertiary L-like structure, which interacts with ribosomes.

The larger loop include a specific nucleotide triplet, the anticodon, wich may bind to a complementary codon of a mRNA.

The stem ends in 3' by the sequence ...CCA, which is the attachment site for an amino acid. Each tRNA is coupled to the amino acid in accordance with its anticodon. The coupling between a given tRNA and the corresponding amino acid is catalyzed by a specific aminoacyl-tRNA synthetases.

The different tRNAs that accept a given amino acid are called isoacceptors.
Obviously, there should be as many different tRNAs as meaning codons (ie 61). In fact there is generally at most 56 different type of tRNAs in any cell. Therefore it seems that some tRNAs are able to recognize at least two of the different codons specifying a given amino acids (Wobble hypothesis).