A polypeptide is a biochemical organic polymer of amino acids connected by covalent peptide bonds. Polypeptides are made through the processes of transcription and translation. In transcription, an enzyme called RNA polymerase forces the double helix of the DNA to unwind, and split in half. Then, the bases of the DNA are paired with their opposite, free floating bases. This creates a new helix, and it splits in half, leaving the original DNA, and a new half string, called messenger RNA, or mRNA. The mRNA leaves the nucleus, and binds with ribosomes. These ribosomes "read" the mRNA, and every three bases, it connects an amino acid using a molecule of transfer RNA (tRNA), building a polypeptide chain. The nascent chain then enters the golgi apparatus where it is further modified by enzymatic cleavage, addition of fatty acids or removal of amino acid side chains. The destination or function of the nascent protein is usually dictated partially by this post-translational modification.