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Are there estimates of the minimum number of genes required to sustain life?

In what I mean by life here, I don't include viruses.

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Interesting question, but you'll need to define needed as well. There are lists of essential genes, those whose loss is deleterious (at least in yeast). If you are looking for the theoretical minimum possible in an imaginary organism, that is a different matter. –  terdon Oct 13 '13 at 22:25
    
Since RNA is the presumed precursor to DNA and might have sustained life before DNA evolved, do RNA counterparts count? –  MCM Oct 14 '13 at 0:44
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This paper, Determination of the Core of a Minimal Bacterial Gene Set, estimate 206 protein coding genes as the minimum genome for a bacterial cell –  TomD Oct 14 '13 at 8:38
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well.. i think if you exclude viruses then you should also exclude mycoplasma because both are obligate parasites. So when you say minimal genome it is essential to define the conditions under which a genome has to function. The retroviral genome is sufficient to sustain life given a host. For free-living organisms the number of genes is obviously higher. –  WYSIWYG Oct 15 '13 at 9:02
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Depends on what kind of life you want! The Database of Essential Genes lists genes essential for life for a number of species, although I would be wary of some of the numbers; bacterial results are probably more reliable but it lists 118 for humans and 2114 for mice. There are a bunch of different numbers for different E. coli strains, with larger numbers being published earlier, go figure.

It of course varies by species. In mid-August of this year, scientists reported the smallest genome yet: 112kb with only 137 protein-coding genes (predicted). Crazy! That doesn't mean it's the smallest number of genes though, just what we've found. Some folks in 2006 predicted the smallest would be 113kb and 151 genes - pretty close close to the above, all in all.

For something a little more familiar and studied, though, Craig Venter has made synthetic biology his goal. Part of trying to create life is knowing what the bare minimum is. Working with Mycoplasma genitalium, he and his crew knocked out each gene and found that 381 were indeed essential.

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