 A DNA sequencing reaction includes 4 main ingredients, template DNA, copied by the E. coli, freebases, the building blocks of DNA that come in 4 types, short pieces of DNA called primers, and DNA polymerase, the enzyme that copies DNA. The chemical reaction that makes DNA in a test tube is similar to what happens in a living cell. Both rely on DNA polymerase, and in both cases DNA strands have a head end, which is called the 5 prime end, and a tail end, which is called the 3 prime end. A DNA strand can grow only from its 3 prime end. Making DNA in cells and sequencing DNA in test tubes both depend on complementary base pairing. The building blocks on opposite strands of DNA pair specifically. A C always pairs with a G, and an A always pairs with a T. The primer sequence binds to its complementary sequence on the template DNA. Freebases that match the template sequence can attach to the new strands growing 3 prime end. Among the freebases in the solution are a few that have a fluorescent dye attached to them. When a dye-bearing base attaches to the growing strand, it stops the new DNA strand from growing any further. A different colored dye is attached to each of the 4 kinds of bases.