DNA replication is a complex process requiring more than a dozen enzymes, nucleotides, and energy. In eukaryotic cells there are multiple sites called origins of replication; at these sites, enzymes unwind the helix by breaking the double bonds between the nitrogen bases. Once the molecule is opened, separate strands are kept from rejoining by DNA stabilizing proteins. DNA polymerase molecules read the sequences in the strands being copied and catalyze the addition of complementary bases to form new strands.