Translation in protein synthesis refers to the phase of protein assembly in cells where RNA is decoded to produce a chain of amino acids. Translation is the second phase of protein production, following transcription, the encoding of DNA into directions for protein assembly in the form of mRNA. The four phases of translation in protein synthesis all occur in the ribosome of the cell, and are called activation, initiation, elongation, and termination. Translation makes the basic structures that underlie much of living tissues, but significant aspects of protein synthesis continue after translation.
Protein synthesis consists of at least two stages. First, in the nucleus of the cell, a strand of the nucleic acid DNA serves as a template to manufacture mRNA, which copies the instructions for the synthesis of amino acids, the building blocks of proteins, from the DNA: this is called transcription. The phase of translation in protein synthesis occurs in the cell but outside the nucleus, in special structures called ribosomes. Translation is the assembly of proteins from the amino acids in a specific order according to the directions of the mRNA.
The mRNA moves out from the nucleus to the cell's ribosomes when translation begins. RNA is organized according to a specific code, where a sequence of three nucleotides is arranged to encode the directions for a corresponding amino acid, a unit called a codon. The ribosome surrounds the mRNA, using it to assemble a chain of amino acids in the same order that they will be found in the finished protein. It forms complexes pairing one amino acid with the appropriate mRNA codon, so the nucleic acid is a blueprint for the finished product. Amino acid synthesis occurs as part of digestion and food metabolism, not translation.
Translation in protein synthesis has several phases, though the process is different in prokaryotic cells — those of bacteria — than in the cells of animals, plants, and fungi. The first phase, activation, pairs amino sequences with their appropriate mRNA codons through chemical bonds in a precise process. Initiation begins on the ribosome when it binds to an initiation site on the mRNA that starts the actual protein assembly. Elongation describes the ribosomal addition of more amino acids to one end of the assembly chain, a process that continues down the mRNA strand until a codon that signifies stop is reached.
The final phase of translation in protein synthesis is called termination, and depends on specialized chemical factors recognizing one of three possible stop messages in the RNA and responding accordingly by releasing the protein from the ribosome. Afterward, the newly assembled protein, called a polypeptide, may be subject to post-translational modification, which includes changes not coded for by the RNA. Furthermore, the polypeptide must be folded into a specific shape — a conformation — that determines the structure and final function of the finished protein.
