An aminoacyl-tRNA synthetase (aaRS or ARS), also called tRNA-ligase, is an enzyme that attaches the appropriate amino acid onto its tRNA. It does so by catalyzing the esterification of a specific cognate amino acid or its precursor to one of all its compatible cognate tRNAs to form an aminoacyl-tRNA. The 20 different types of aa-tRNA are made by the 20 different aminoacyl-tRNA synthetases, one for each amino acid of the genetic code. This process is sometimes called "charging" or "loading" the tRNA with the amino acid. Once the tRNA is charged, a ribosome can transfer the amino acid from the tRNA onto a growing peptide, according to the genetic code. Aminoacyl-tRNA, therefore, plays an important role in translation, the expression of genes to create proteins. Translation is carried out by ribosomes in the cytoplasm or endoplasmic reticulum after the process of transcription of DNA to RNA in the cell's nucleus (Wikipedia).

tRNA Charging: Serine References