The answer is DNA joins another DNA molecule from Intermolecular Hydrogen Bonds. Adenine forms two Hydrogen bonds with Thymine.
Cytosine forms three Hydrogen bonds with Guanine.
At each attachment point within a polynucleotide, the 5' end of one nucleotide joins the 3' end of the adjacent nucleotide through a linkage called a phosphodiester bond (Figure 3). It is this alternating arrangement of sugars and phosphate that forms the "backbone" of a DNA molecule.
What is DNA replication?
Before a cell divides, its DNA replicates (duplicates). Because the two strands of a DNA molecule have complementary base pairs, the nucleotide sequence of each strand automatically provides the information needed to make its partner. When the two strands of a DNA molecule are separated, each can be used as a template to make a complementary strand. Each template and its new complement together form a new DNAdouble helix identical to the original one.
Before replication can take place, the length of the double helix of the DNA to be copied must be unwound. Also, the two strands must pull apart like the two sides of a zipper, breaking the weak hydrogen bonds that connect the paired bases. Once the DNA strands have unwound, they must be held apart to expose the bases so new nucleotides can form hydrogen bonds with them.
The enzyme DNA polymerase then moves along the exposed strand of DNA, joining the newly arrived nucleotides into a new strand of DNA that is complementary to the template.
