Cycle Sequencing

To sequence a piece of dna you need 1)a Template DNA 2) a short DNA primer that is complementary to the dna you want to sequence, 3)A enzyme called DNA polymerase,(4) Four nucleotides.(A,C,G,T), To this mix ,we also add a second type of nucleotide; one that has a slightly different chemical formula, These dideoxynucleotides(diddtp) can be recognized by a DNA sequencer.

To start the sequencing reaction this mixture is heated to 96C ,so the template DNA's two complementary strand separates,Then the temperature is lowered, so that the short "primer" sequence finds its complementary sequence in the template DNA.Finally the temperature is raised 60c,this allows the enzyme to bind to the DNA and create a new strand of DNA.

The sequence of this new DNA is complementary to the original DNA strand. The enzyme makes no distinction between dNTPs or didNTPs.each time a didNTP is incorporated, in this case didATP,The synthesis stops. Because billion of DNA molecules are present in the test tube, the strand can be terminated at any position. This results in collection of DNA strands of many different lengths.

The sequencing reaction is transferred from the test tube to a lane of a polyacrylamide gel. The gel is placed into a DNA sequencer for electrophoresis and analysis. The fragments migrate according to size and each is detected as it passes a laser beam at the bottom of the gel. Each type of dideoxynucleotide emits colored light of a characteristic wavelength and is recorded as a colored band on a simulated gel image, and finally computer program interprets the raw data and outputs an electropherogram with colored peaks representing each letter in the sequence.the sequence fragments are sorted out according to the size, starting from the shortest to longest one, the stimulated gel image is read from bottom to top, starting with the smallest fragment, Thus we sequences present in template DNA.