In chain termination sequencing we synthesize new strands of DNA which are complementary to a single-stranded template. The template DNA is provided with a mixture consisting of all four deoxynucleotides with each of them labeled with a different colored fluorescent tag and DNA polymerase. The presence of all four deoxynucleotides allows chain elongation to proceed till dideoxynucleotide is inserted by the DNA polymerase. The outcome of this is a new set of DNA chains all of which have different lengths. Using gel electrophoresis the fragments are separated based on their size. In the detector as the labeled DNA fragments pass a detector at the bottom of the gel, the color is recorded. The different colors which represent the various nucleotide sequences allow the DNA sequence to be reconstructed from their pattern.