Figure 4. Two frames from the “Chemistry of Sequencing” animation. (a) An overview of the dye terminator reaction in DNA sequencing. Polymerase is shown as an orange oval traveling along the growing DNA strand. The bases are color-coded rectangles. The dye terminators are shown as bases with a brightly colored circle at one end. (b) A growing DNA strand in a sequencing reaction. The arrow points to a 3′ hydroxyl group of a ribose (oxygen is shown as a yellow sphere, and hydrogen is shown as a pink sphere). A nucleotide is shown approaching the 3′ hydroxyl. In the next step (not shown here), the new bond that is formed between the 3′ hydroxyl group and the 5′ α-phosphate of the nucleotide is shown.