DNA replication begins at the origin of replication where DNA helicase unwinds and unzips the double helix. DNA polymerase reads the bases on one strand and adds complementary bases to the other strand. The leading strand is replicated continuously while the lagging strand is replicated discontinuously in fragments called Okazaki fragments. DNA primase adds primers to fill in the lagging strand, and DNA ligase seals the fragments together with phosphodiester bonds.