Editing Origin of replication (section)

== Variations ==
Although DNA replication is essential for genetic inheritance, defined, site-specific replication origins are technically not a requirement for genome duplication as long as all chromosomes are copied in their entirety to maintain gene copy numbers. Certain bacteriophages and viruses, for example, can initiate DNA replication by homologous recombination independent of dedicated origins.<ref name="#9928485">{{cite journal | vauthors = Mosig G | title = Recombination and recombination-dependent DNA replication in bacteriophage T4 | journal = Annual Review of Genetics | volume = 32 | pages = 379–413 | pmid = 9928485 | doi = 10.1146/annurev.genet.32.1.379 | year = 1998 }}</ref> Likewise, the archaeon ''[[Haloferax volcanii]]'' uses recombination-dependent initiation to duplicate its genome when its endogenous origins are deleted.<ref name="Hawkins_2013"/> Similar non-canonical initiation events through break-induced or transcription-initiated replication have been reported in ''E. coli'' and ''S. cerevisiae''.<ref name="#28134821">{{cite journal | vauthors = Ravoitytė B, Wellinger RE | title = Non-Canonical Replication Initiation: You're Fired! | journal = Genes | volume = 8 | issue = 2 | pages = 54 | date = January 2017 | pmid = 28134821 | pmc = 5333043 | doi = 10.3390/genes8020054 | doi-access = free }}</ref><ref name="#8344265">{{cite journal | vauthors = Asai T, Sommer S, Bailone A, Kogoma T | title = Homologous recombination-dependent initiation of DNA replication from DNA damage-inducible origins in Escherichia coli | journal = The EMBO Journal | volume = 12 | issue = 8 | pages = 3287–95 | date = August 1993 | pmid = 8344265 | pmc = 413596 | doi = 10.1002/j.1460-2075.1993.tb05998.x }}</ref><ref name="#17671506">{{cite journal | vauthors = Lydeard JR, Jain S, Yamaguchi M, Haber JE | title = Break-induced replication and telomerase-independent telomere maintenance require Pol32 | journal = Nature | volume = 448 | issue = 7155 | pages = 820–3 | date = August 2007 | pmid = 17671506 | doi = 10.1038/nature06047 | bibcode = 2007Natur.448..820L | s2cid = 4373857 }}</ref><ref name="#2446774">{{cite journal | vauthors = Dasgupta S, Masukata H, Tomizawa J | title = Multiple mechanisms for initiation of ColE1 DNA replication: DNA synthesis in the presence and absence of ribonuclease H | journal = Cell | volume = 51 | issue = 6 | pages = 1113–22 | date = December 1987 | pmid = 2446774 | doi = 10.1016/0092-8674(87)90597-6 | s2cid = 22858038 }}</ref><ref name="#25902524">{{cite journal | vauthors = Stuckey R, García-Rodríguez N, Aguilera A, Wellinger RE | title = Role for RNA:DNA hybrids in origin-independent replication priming in a eukaryotic system | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 18 | pages = 5779–84 | date = May 2015 | pmid = 25902524 | pmc = 4426422 | doi = 10.1073/pnas.1501769112 | bibcode = 2015PNAS..112.5779S | doi-access = free }}</ref> Nonetheless, despite the ability of cells to sustain viability under these exceptional circumstances, origin-dependent initiation is a common strategy universally adopted across different domains of life.<ref name="Ekundayo et al"/>

In addition, detailed studies of replication initiation have focused on a limited number of model systems. The extensively studied fungi and metazoa are both members of the [[opisthokont]] supergroup and exemplify only a small fraction of the evolutionary landscape in the eukaryotic domain.<ref name="#24789819">{{cite journal | vauthors = Burki F | title = The eukaryotic tree of life from a global phylogenomic perspective | journal = Cold Spring Harbor Perspectives in Biology | volume = 6 | issue = 5 | pages = a016147 | date = May 2014 | pmid = 24789819 | pmc = 3996474 | doi = 10.1101/cshperspect.a016147 }}</ref> Comparably few efforts have been directed at other eukaryotic model systems, such as kinetoplastids or [[tetrahymena]].<ref name="#25569357">{{cite journal | vauthors = Lee PH, Meng X, Kapler GM | title = Developmental regulation of the Tetrahymena thermophila origin recognition complex | journal = PLOS Genetics | volume = 11 | issue = 1 | pages = e1004875 | date = January 2015 | pmid = 25569357 | pmc = 4287346 | doi = 10.1371/journal.pgen.1004875 | doi-access = free }}</ref><ref name="#18007594">{{cite journal | vauthors = Mohammad MM, Donti TR, Sebastian Yakisich J, Smith AG, Kapler GM | title = Tetrahymena ORC contains a ribosomal RNA fragment that participates in rDNA origin recognition | journal = The EMBO Journal | volume = 26 | issue = 24 | pages = 5048–60 | date = December 2007 | pmid = 18007594 | pmc = 2140106 | doi = 10.1038/sj.emboj.7601919 }}</ref><ref name="#19153611">{{cite journal | vauthors = Donti TR, Datta S, Sandoval PY, Kapler GM | title = Differential targeting of Tetrahymena ORC to ribosomal DNA and non-rDNA replication origins | journal = The EMBO Journal | volume = 28 | issue = 3 | pages = 223–33 | date = February 2009 | pmid = 19153611 | pmc = 2637336 | doi = 10.1038/emboj.2008.282 }}</ref><ref name="#29491738">{{cite journal | vauthors = Marques CA, McCulloch R | title = Conservation and Variation in Strategies for DNA Replication of Kinetoplastid Nuclear Genomes | journal = Current Genomics | volume = 19 | issue = 2 | pages = 98–109 | date = February 2018 | pmid = 29491738 | pmc = 5814967 | doi = 10.2174/1389202918666170815144627 }}</ref><ref name="#26951375">{{cite journal | vauthors = Marques CA, Tiengwe C, Lemgruber L, Damasceno JD, Scott A, Paape D, Marcello L, McCulloch R | display-authors = 6 | title = Diverged composition and regulation of the Trypanosoma brucei origin recognition complex that mediates DNA replication initiation | journal = Nucleic Acids Research | volume = 44 | issue = 10 | pages = 4763–84 | date = June 2016 | pmid = 26951375 | pmc = 4889932 | doi = 10.1093/nar/gkw147 }}</ref><ref name="#22412905">{{cite journal | vauthors = Tiengwe C, Marcello L, Farr H, Gadelha C, Burchmore R, Barry JD, Bell SD, McCulloch R | display-authors = 6 | title = Identification of ORC1/CDC6-interacting factors in Trypanosoma brucei reveals critical features of origin recognition complex architecture | journal = PLOS ONE | volume = 7 | issue = 3 | pages = e32674 | pmid = 22412905 | pmc = 3297607 | doi = 10.1371/journal.pone.0032674 | year = 2012 | bibcode = 2012PLoSO...732674T | doi-access = free }}</ref><ref name="#26481451">{{cite journal | vauthors = Marques CA, Dickens NJ, Paape D, Campbell SJ, McCulloch R | title = Genome-wide mapping reveals single-origin chromosome replication in Leishmania, a eukaryotic microbe | journal = Genome Biology | volume = 16 | pages = 230 | date = October 2015 | pmid = 26481451 | pmc = 4612428 | doi = 10.1186/s13059-015-0788-9 | doi-access = free }}</ref> Surprisingly, these studies have revealed interesting differences both in origin properties and in initiator composition compared to yeast and metazoans.<ref name="Ekundayo et al"/>