After the mRNA transcript is synthesized and processed in the nucleus, it enters the cytoplasm. The next step is translation, the assembly of amino acids into proteins in ribosomes through the reading of mRNA by tRNA and the ribosome. In the cytoplasm, ribosomes, which are small complexes of RNA and proteins, bind the mRNA transcript. Ribosomal RNA (rRNA) is the RNA component of ribosomes that catalyzes peptide bond formation. A molecule that carries each amino acid to the strand of mRNA during translation of protein synthesis is called transfer RNA (tRNA).The tRNA has a specific shape that helps it bind to its corresponding amino acid. The sequence of tRNA causes it to form hydrogen bonds between its nucleotides and modified bases, resulting in a cloverlike secondary structure with three loops. One of these loops contains the anticodon, a sequence of three nucleotides of a tRNA molecule that pairs with the complement on an mRNA strand during protein synthesis. On the other end of the tRNA, the enzyme aminoacyl-tRNA synthetase attaches an amino acid to a tRNA molecule. This is known as "loading" or "charging" the tRNA. The choice of which of the twenty amino acids to add to the tRNA is determined by the codon/anticodon sequence according to the universal genetic code. The loaded tRNA can then take the amino acid to the ribosome to build the mRNA. The three-letter-codon genetic code is referred to as "universal" because, aside from a few minor differences in a small percentage of organisms, the same codons direct the assembly of the same amino acids into proteins of practically every living creature on Earth. The ribosome is composed of two subunits. The small subunit binds the 5′ cap on the mRNA, while the large subunit binds the tRNA and associated amino acid. The first codon (three-base mRNA sequence associated with an amino acid) is the start codon, which corresponds to methionine, a sulfur-containing amino acid that is an essential part of many proteins. Translation begins with an initiator tRNA, which delivers a methionine, the first amino acid. The ribosome binding site where a charged tRNA associated with an amino acid first attaches is called the A site. The A is for aminoacyl tRNA. It then moves to the second site, the P site, or the ribosome binding site where an amino acid is added to the growing peptide chain. The P stands for peptidyl tRNA. A peptide chain is a series of amino acids linked together. The final tRNA binding site is the E site, the ribosome binding site where the tRNA is ejected following translation. The E stands for exit. However, when this process begins, the first methionine actually sits in the P site.