A DNA primer complementing the template DNA (the DNA to be sequenced) is used in Sanger sequencing to be a starting point for DNA synthesis. In the presence of the four deoxynucleotide triphosphates (dNTPs: A, G, C, and T), the polymerase expands the primordial by adding the additional dNTP to the DNA strand of the template. Four dideoxynucleotide triphosphates (ddNTPs: ddATP, ddGTP, ddCTP, and ddTTP) labeled with a distinct fluorescent dye are used to terminate the synthesis process to establish which nucleotide is incorporated in the nucleotide chain. Compared to dNTPs, ddNTPs has a removed oxygen atom from the ribonucleotide, so it can not form a connection to the next nucleotide. Upon synthesis, the reaction products are loaded into four lanes of a single gel, depending on the different chain-terminating nucleotide and are subjected to gel electrophoresis. Thus the DNA sequence is determined according to their sizes.