Protein synthesis is the building of proteins, essential organic compounds for intra- and extracellular functions. Proteins are molecules that provide structure and support for living things, as well as catabolizing chemical reactions in their enzymatic roles.
In the cell, the instructions for building the myriad of proteins needed by living things is encoded on the DNA. The DNA is stored in the nucleus of the cell for safekeeping, and the code on the DNA must be copied and translated by other molecules to build the protein.
From hair color to addictive tendencies, DNA has genes on it coding for the development of proteins. Genes begin with a start sequence or codon, and terminate with one of three stop codons. When a protein is needed, a molecule known as messenger RNA, or mRNA is formed beginning at the start codon and terminating at the stop codon. This mRNA molecule is the transcriber, copying DNA's coded instructions for the protein and carrying them out of the cell to the assembly structures. This step of protein synthesis, therefore, is known as transcription.
When the mRNA leaves the nucleus, it enters the cytoplasm of the cell. In the cytoplasm, structures known as ribosomes are responsible for binding to the mRNA in order for the second step, translation of the code and assembly of the protein, to occur. Ribosomes consist of two subunits that clamp down around an mRNA strand. The units have binding sites for other RNA molecules, known as tRNA's (transfer RNA's) to bring over amino acids in sequence, matching the coded instructions on the mRNA molecule. These binding sites are known as the A site and the P site.
In translation, the mRNA molecule's code is read and the protein is assembled by stringing together the amino acids in sequence, beginning at the start codon and ending at the stop codon. Once the protein is built, the ribosome unclamps from the mRNA molecule, the finished protein is free to be used where it is needed, and the mRNA can be translated once again until the cell no longer requires the protein its instructions code for.