The process whereby DNA makes exact copies of itself, which is controlled by the enzyme DNA polymerase. Replication occurs at rates of between 50 nucleotides per second (in mammals) and 500 nucleotides per second (in bacteria). The hydrogen bonds between the complementary bases on the two strands of the parent DNA molecule break and the strands unwind, forming a Y-shaped replication fork. Each strand acts as a template for the synthesis of a new one complementary to itself (see dna-binding proteins; primosome). DNA polymerases move along the two single strands linking free nucleotides to their complementary bases (see base pairing) on the templates. The process continues until all the nucleotides on the templates have joined with appropriate free nucleotides and two identical molecules of DNA have been formed. This process is known as semiconservative replication as each new molecule contains half of the original parent DNA molecule (compare conservative replication; dispersive replication). DNA polymerases can add nucleotides only to the 3´ end of the growing DNA strand; hence elongation occurs in the 5´ to 3´ direction, thus preserving the antiparallel orientation of the new strand to the template strand (see discontinuous replication). Sometimes mutations occur that may cause the exact sequence of the parent DNA not to be replicated. However, DNA repair mechanisms reduce this possibility. In bacteria and other prokaryotes, replication starts with the binding of a pre-replication complex of proteins, including DNA polymerase, at a specific site (called ori, for origin of replication) on the chromosome. In eukaryotes the various proteins involved in DNA replication form a replication complex, multiple copies of which may be present in the nuclear matrix. Replication is initiated at numerous ori sites along each DNA molecule and proceeds simultaneously in both directions at each site.