In the living cell, however, DNA is not formed as a free strand in isolation, but on a template formed by a preexisting DNA strand. The bases that stick out from this template can bind to bases of the strand being formed, according to a strict rule defined by the complementary structures of the bases: A binds to T, and C binds to G. This base-pairing holds fresh monomers in place and thereby controls the selection of which one of the four monomers will next be added to a growing strand. In this way, a double-strand structure is created, consisting of two exactly complementary sequences of A’s, C’s, T’s, and G’s. These two strands twist around each other, forming a DNA double helix.
