= Haloferax volcanii
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<Haloferax volcanii> is a salt-requiring member of <archaea> and a useful <model organism> for experimental archaeal biology. As a <halophile>, it grows in saline conditions; its ability to grow in defined laboratory media and undergo genetic manipulation lets investigators connect specific <DNA> changes to cellular behavior.<The Complete Genome Sequence of Haloferax volcanii DS2, a Model Archaeon by Amber L. Hartman and others>[Abstract and Introduction: culture and genetic tractability]{ref}
A 2010 <genome assembly> of strain DS2 described a main <chromosome>, three smaller chromosomes and a small <plasmid>. Distinguishing different DNA elements from multiple copies of an element is important: four chromosome types do not, by themselves, specify how many genome copies one cell carries.<The Complete Genome Sequence of Haloferax volcanii DS2, a Model Archaeon by Amber L. Hartman and others>[Abstract; Results,Genome features and chromosome nomenclature]{ref}
A striking 2013 experiment used laboratory strain H26 to test whether defined <origin of replication> sites were required. Deleting individual chromosomal origins slowed growth, but deleting all four produced viable cells that grew faster in the tested comparison. Sequencing profiles indicated dispersed initiation rather than replacement by one new sharp origin; the origin-deleted cells depended on <homologous recombination>.<Accelerated growth in the absence of DNA replication origins by Michelle Hawkins and others>[author manuscript,pp1–3;Figures2–4]{ref}
The experiment separates a process from one way of organizing it. Removing defined start sites need not abolish <DNA replication> if another initiation mechanism is available. The result concerns this engineered archaeal strain under the studied conditions, rather than proving that replication origins are dispensable in every organism.
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