During replication of the DNA molecule, each strand serves as a template for the assembly of a new strand. The DNA strands separate, and free, unattached nucleotides associate with their complementary bases through hydrogen bonding. DNA polymerase, the enzyme that adds nucleotides to the newly-forming strand, can add them only at the 3' end. Ultimately, two DNA duplexes are formed, each containing one old strand and one newly formed strand. This is called semiconservative replication.