According to this paper, the ATP cost of a having (not counting transcribing) a gene in a diploid eukaryote is about $5\times 10^3$ ATP per base pair, while the lifetime ATP usage of a mono-cellular eukaryote might be around $10^{9+3}=10^{12}$ ATP. The human genome contains about $3\times 10^9$ base pairs, and all taken together this implies that the cost to the cell in a multicellular organism of copying its genome may be a respectable fraction of its total lifetime energy costs. (I am not willing to say anything more specific than that, because these are ballpark numbers and vary widely from organism or organism and even cell to cell.)

The process of cell differentiation inactivates large blocks of genes. This paper suggests that only ~6% of mammalian genes are ubiquitously expressed, while each tissue tends to express only 30-40% of the whole genome it carries.

Taken together, these two facts imply that an organism whose cells did not copy permanently inactivated genes could save as much as half the energy it would have to spend otherwise on genetic replication, which in the first paragraph I argued was not insignificant. Nonetheless, all of the organisms I can think of put full copies of their entire genomes in every cell that will ever need to multiply (mature red blood cells do not contain copies of the full genome, but that is only because they do not have a nucleus). Are there any species that exhibit this parsimony?

I recognize that a mechanism for doing this would be nontrivial, because some genes are inactive during differentiation but must be present to become active later, but life, uhh, finds a way.

  • $\begingroup$ To put these numbers in context, the human body has a daily ATP budget on the order of 100 moles. Translating that to numbers of ATP molecules ($\approx10^{26}$), the quantities you cite are minuscule. Looking at figure 3 of the paper you link, those estimates are more burdensome (accounting for the whole genome) but are still tiny as a fraction of the energy budget (bottom axis). $\endgroup$ May 28 at 4:31
  • $\begingroup$ Also: you might be interested in the ciliate micronucleus/macronucleus organization as a related strategy. $\endgroup$ May 29 at 4:27
  • $\begingroup$ @MaximilianPress The ATP budget of the whole body is not a good number to compare the costs of a single cell with. There is a lot of ATP available every day, but there are also a lot of cell divisions. $\endgroup$
    – Retracted
    May 30 at 18:20
  • $\begingroup$ Fair enough, to me it was not clear which those numbers referred to, though going back and rereading I see it now. Also, only just noticed: you write $10^{9+3} = 10^{11}$ where I'm pretty sure it should be $10^{12}$. I still think that Figure 3 pretty much has the answer, which is the same: this is not a very large cost for selection except at astronomical population sizes (bacteria), which have quite compact genomes already (possibly related). $\endgroup$ May 31 at 15:37


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