Editing DNA (section)

=== Replication ===
{{further|DNA replication}}
[[File:DNA replication en.svg|thumb|upright=1.78|right|DNA replication: The double helix is unwound by a [[helicase]] and [[topoisomerase|topo&shy;iso&shy;merase]]. Next, one [[DNA polymerase]] produces the [[Replication fork|leading strand]] copy. Another DNA polymerase binds to the [[Replication fork|lagging strand]]. This enzyme makes discontinuous segments (called [[Okazaki fragment]]s) before [[DNA ligase]] joins them together.]]
[[Cell division]] is essential for an organism to grow, but, when a cell divides, it must replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent. The double-stranded structure of DNA provides a simple mechanism for [[DNA replication]]. Here, the two strands are separated and then each strand's [[complementary DNA]] sequence is recreated by an [[enzyme]] called [[DNA polymerase]]. This enzyme makes the complementary strand by finding the correct base through complementary base pairing and bonding it onto the original strand. As DNA polymerases can only extend a DNA strand in a 5′ to 3′ direction, different mechanisms are used to copy the antiparallel strands of the double helix.<ref>{{cite journal | vauthors = Albà M | title = Replicative DNA polymerases | journal = Genome Biology | volume = 2 | issue = 1 | pages = REVIEWS3002 | year = 2001 | pmid = 11178285 | pmc = 150442 | doi = 10.1186/gb-2001-2-1-reviews3002 | doi-access = free }}</ref> In this way, the base on the old strand dictates which base appears on the new strand, and the cell ends up with a perfect copy of its DNA.