Editing DNA (section)

==== Polymerases ====
[[Polymerase]]s are [[enzyme]]s that synthesize polynucleotide chains from [[nucleoside triphosphate]]s. The sequence of their products is created based on existing polynucleotide chains—which are called ''templates''. These enzymes function by repeatedly adding a nucleotide to the 3′ [[hydroxyl]] group at the end of the growing polynucleotide chain. As a consequence, all polymerases work in a 5′ to 3′ direction.<ref name=Joyce>{{cite journal | vauthors = Joyce CM, Steitz TA | title = Polymerase structures and function: variations on a theme? | journal = Journal of Bacteriology | volume = 177 | issue = 22 | pages = 6321–29 | date = November 1995 | pmid = 7592405 | pmc = 177480 | doi=10.1128/jb.177.22.6321-6329.1995}}</ref> In the [[active site]] of these enzymes, the incoming nucleoside triphosphate base-pairs to the template: this allows polymerases to accurately synthesize the complementary strand of their template. Polymerases are classified according to the type of template that they use.

In DNA replication, DNA-dependent [[DNA polymerase]]s make copies of DNA polynucleotide chains. To preserve biological information, it is essential that the sequence of bases in each copy are precisely complementary to the sequence of bases in the template strand. Many DNA polymerases have a [[Proofreading (biology)|proofreading]] activity. Here, the polymerase recognizes the occasional mistakes in the synthesis reaction by the lack of base pairing between the mismatched nucleotides. If a mismatch is detected, a 3′ to 5′ [[exonuclease]] activity is activated and the incorrect base removed.<ref>{{cite journal | vauthors = Hubscher U, Maga G, Spadari S | s2cid = 26171993 | title = Eukaryotic DNA polymerases | journal = Annual Review of Biochemistry | volume = 71 | pages = 133–63 | year = 2002 | pmid = 12045093 | doi = 10.1146/annurev.biochem.71.090501.150041 | url = http://pdfs.semanticscholar.org/e941/98efed7eb8fa606b87d9a44c118c235a62e9.pdf | archive-url = https://web.archive.org/web/20210126170051/http://pdfs.semanticscholar.org/e941/98efed7eb8fa606b87d9a44c118c235a62e9.pdf | url-status = dead | archive-date = 26 January 2021 }}</ref> In most organisms, DNA polymerases function in a large complex called the [[replisome]] that contains multiple accessory subunits, such as the [[DNA clamp]] or [[helicase]]s.<ref>{{cite journal | vauthors = Johnson A, O'Donnell M | title = Cellular DNA replicases: components and dynamics at the replication fork | journal = Annual Review of Biochemistry | volume = 74 | pages = 283–315 | year = 2005 | pmid = 15952889 | doi = 10.1146/annurev.biochem.73.011303.073859 }}</ref>

RNA-dependent DNA polymerases are a specialized class of polymerases that copy the sequence of an RNA strand into DNA. They include [[reverse transcriptase]], which is a [[virus|viral]] enzyme involved in the infection of cells by [[retrovirus]]es, and [[telomerase]], which is required for the replication of telomeres.<ref name=Greider /><ref name=Tarrago-Litvak1994>{{cite journal | vauthors = Tarrago-Litvak L, Andréola ML, Nevinsky GA, Sarih-Cottin L, Litvak S | title = The reverse transcriptase of HIV-1: from enzymology to therapeutic intervention | journal = FASEB Journal | volume = 8 | issue = 8 | pages = 497–503 | date = May 1994 | pmid = 7514143 | doi = 10.1096/fasebj.8.8.7514143 | doi-access = free | s2cid = 39614573 }}</ref> For example, HIV reverse transcriptase is an enzyme for AIDS virus replication.<ref name=Tarrago-Litvak1994 /> Telomerase is an unusual polymerase because it contains its own RNA template as part of its structure. It synthesizes [[telomeres]] at the ends of chromosomes. Telomeres prevent fusion of the ends of neighboring chromosomes and protect chromosome ends from damage.<ref name=Nugent />

Transcription is carried out by a DNA-dependent [[RNA polymerase]] that copies the sequence of a DNA strand into RNA. To begin transcribing a gene, the RNA polymerase binds to a sequence of DNA called a promoter and separates the DNA strands. It then copies the gene sequence into a [[messenger RNA]] transcript until it reaches a region of DNA called the [[terminator (genetics)|terminator]], where it halts and detaches from the DNA. As with human DNA-dependent DNA polymerases, [[RNA polymerase II]], the enzyme that transcribes most of the genes in the human genome, operates as part of a large [[protein complex]] with multiple regulatory and accessory subunits.<ref>{{cite journal | vauthors = Martinez E | s2cid = 24946189 | title = Multi-protein complexes in eukaryotic gene transcription | journal = Plant Molecular Biology | volume = 50 | issue = 6 | pages = 925–47 | date = December 2002 | pmid = 12516863 | doi = 10.1023/A:1021258713850 }}</ref>