Aminoacyl transfer ribonucleic acid (aminoacyl tRNA) is used in translating sequences of mRNA into proteins. Aminoacyl tRNA consists of a strand of RNA that includes a group of three nucleotides, called a codon, joined to an amino acid. Each codon is paired to a specific amino acid, although there is some redundancy; some amino acids are paired with multiple codons. This form of tRNA helps to transport amino acids to the ribosome, where translation takes place, and the tRNA codon then pairs with a complementary sequence on the mRNA strand, allowing its cognate amino acid to join the polypeptide chain formed during translation. Through this process, genetic information that was originally contained in the strand of tRNA can be converted into amino acids that are used to form proteins.
After tRNA molecules are transcribed from the DNA sequence that codes for them, there is a two step process to convert these tRNA strands into aminoacyl tRNA molecules. These reactions take place inside of the specific aminoacyl tRNA synthetase enzyme for a given amino acid. There are 20 types of these enzymes in all, one for each amino acid.
Initially, the amino acid that is to be paired for the tRNA sequence must be activated. This is accomplished by adenylating the amino acid, or binding it to an adenosine monophosphate (AMP) molecule in an energy-consuming reaction. The tRNA then transfers to the amino acid-AMP complex, and removes the AMP in order to join to the amino acid. The AMP in this reaction comes from adenosine triphosphate (ATP), which converts to AMP and a pyrophosphate molecule that provides the energy for this reaction.
Aminoacyl tRNA synthetase enzymes are macromolecules that recognize which tRNA sequences pair to the correct amino acid in a couple of different ways. The enzymes have anticodon regions of their own tRNA, which can recognize the sequences of tRNA codons. Alternately, the enzyme can recognize acceptor sites on tRNA sequences that are located at either end of the molecules.
These multiple recognition sites ensure that amino acids are paired to the correct tRNA sequences, and are particularly important for amino acids like serine, which can match to six different codons of tRNA. The tRNA sequences also contain genetic information aside from a codon and acceptor sites. There are discriminator bases around the codon that prevent the wrong aminoacyl tRNA synthetase enzyme from taking it up and using it in a reaction.