30. Two phases of protein synthesis are transcription, or the copying of the DNA information into the messenger RNA and translation, where the information carried by mRNA is decoded and used to assemble polypeptides. In transcription, information is transferred from a DNA gene's base sequence to the complementary base sequence of an mRNA molecule. Once the mRNA molecule is made, it detaches and leaves the nucleus via a nuclear pore. Only DNA and mRNA are involved in the transcription process. In translation, the language of nucleic acids is translated into the language of proteins. The process of translation occurs in the cytoplasm and involves all three varieties of RNA. When it reaches the cytoplasm, the mRNA molecule carrying instructions for a particular protein binds to a small ribosomal subunit by base pairing to rRNA. Then tRNA has to transfer the ribosome. Once its amino acid is loaded, the tRNA migrates to the ribosome where it maneuvers the amino acid into the proper position, as said to by the mRNA codons.The ribosome has three binding sites for tRNA: an A (aminoacy) site for incoming tRNA, a P (peptidyl) site for the tRNA holding the growing polypeptide chain, and an E (exit) site for outgoing tRNA. The ribosome holds the tRNA and the mRNA close together to coordinate the coupling of codons and anticodons, and positions the next amino acid for addition to the growing polypeptide chain. The mRNA attaches to the small ribosomal subunit by a leader sequence of bases that functions only to make the attachment. When the initiator tRNA binds to the mRNA, it occupies the P site on the ribosome and the A and E sites are vacant. Then the ribosome slides and brings the next codon into position to be read by tRNA.Once the translation of the mRNA is finished, the ribosome can split into its two subunits. Each sequence of three bases on DNA is called a triplet. The sequence of triplets in each gene forms a "sentence" that tells exactly how a particular polypeptide is to be made. It specifies the number, kinds, and order of amino acids needed to build a particular polypeptide. For each triplet on DNA, the corresponding three-base sequence on mRNA is a codon. Since there are four kinds of RNA (or DNA) nucleotides, there are 64 codons. These are "stop signs" that call for termination of a polypeptide. The head end of tRNA is called an anticodon, a three-base sequence complementary to the mRNA codon calling for the amino acid carried by that particular tRNA. Anticodons form hydrogen bonds with complementary codons, meaning that a tRNA is the link between the language of nucleic acids and the language of proteins.