The document describes the process of DNA replication. It begins with DNA unwinding at the origin of replication site, where helicase enzymes cause the double helix to separate. Free nucleotides then base pair with the exposed, complementary bases on each single strand. DNA polymerase joins the nucleotides to form new polynucleotide chains. Finally, the two new DNA molecules each contain one original and one new strand, and the double helix reforms.