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Which animal/plant/anything has smallest length genome?

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Since you said plant/animal/anything, I offer the smallest genomes in various categories...

(Kb means Kilobases, Mb means Megabases. 1 Kb = 1000 base pairs, 1Mb = 1000Kb)

Refs...

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    $\begingroup$ Well done! How did you find these? $\endgroup$ – user560 Mar 12 '12 at 16:36
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    $\begingroup$ The plant and bacterial ones I had floating around in my memory having read about them previously, so a bit of googling confirmed the names. The animal ones I got from the Animal Genome Size DB, and from Wikipedia. I guessed that viral genomes would be the smallest, so I also searched for the smallest viral genome. Then I went back to the literature (by searching Google Scholar for organism name + "genome size") to check all my facts and provide reliable references. $\endgroup$ – Rik Smith-Unna Mar 12 '12 at 18:43
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    $\begingroup$ Nice answer Richard. $\endgroup$ – Poshpaws Mar 13 '12 at 11:38
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I want to say Mycoplasma genitalium with a genome size of 582,970 bp. Turns out the answer is Nanoarchaeum eqitans with a genome of 490,885 bp.

http://en.wikipedia.org/wiki/Nanoarchaeum

http://www.ncbi.nlm.nih.gov/pubmed/14566062

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    $\begingroup$ That's the smallest genome of a free-living organism. And until 2003 you would have been right about Mycoplasma, which were previously the smallest known in that category. $\endgroup$ – Rik Smith-Unna Mar 12 '12 at 0:38
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Both Mycoplasma genitalium and Nanoarchaeum equitans are obligate parasites / endosymbionts. This means that they depend heavily on their host to support their vital functions and they have lost many of their own genes.

A really free-living organism with an extremely small genome (~1309 kbp, 1354 genes) is the heterotrophic marine alpha-proteobacterium Pelagibacter ubique [1].

See a larger analysis here: https://alamot.github.io/smallest_genome/

[1]: https://www.ncbi.nlm.nih.gov/pubmed/16109880 Giovannoni SJ, Tripp HJ, Givan S, Podar M, Vergin KL, et al. (2005). «Genome streamlining in a cosmopolitan oceanic bacterium». Science 309: 1242–1245.

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Smallest free-living bacterial genome: Nanoarchaeum eqitans at 491Kb (Waters et al., 2003) I downloaded this paper it says that this is archaea and its obligate symbiont not a free living bacteria. Please read carefully and then upload it.

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    $\begingroup$ Can you please edit your answer and add a link for the reference? $\endgroup$ – Chris Aug 14 '15 at 10:16

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