The rule of base pairing helps explain how DNA is replicated prior to cell division. Enzymes unzip the DNA by breaking the hydrogen bonds between the base pairs. The unpaired bases are now free to bind with other nucleotides with the appropriate complementary bases. The enzyme primase begins the process by synthesizing short primers of RNA nucleotides complementary to the unpaired DNA. DNA polymerase now attaches DNA nucleotides to one end of the growing complementary strand of nucleotides. Replication proceeds continuously along one strand, called the leading strand, which is shown here on the left. The process occurs in separate short segments called Okazaki fragments next to the other, or lagging, strand on the right. This difference is due to the fact that DNA polymerase can only add new nucleotides to the 3 prime end of a nucleotide strand. A primer begins any new strand, including each Okazaki fragment. An enzyme replaces the RNA primer with DNA nucleotides. Then an enzyme called DNA ligase binds the fragments to one another. There are now two DNA molecules. Each consists of an original nucleotide strand next to a new complementary strand. The two molecules are identical to each other.