The ribosomes in the cytoplasm are the sites of protein translation in the cell. Translation refers to taking a copy of messenger RNA, or mRNA, and making a protein from the code.
At the start of translation, the ribosomal subunits assemble to form the ribosome. The mRNA strand has a 5' and a 3' end. The 5' end of the mRNA strand attaches itself to the ribosome. The mRNA is read, or translated, three codons at a time. Each set of three codons stands for a certain amino acid. Transfer RNA, or tRNA, molecules, which carry amino acids, bring their amino acid loads to add to the growing protein chain. Amino acids are the subunits, or building blocks, of proteins.