This is both a math and biology question but I think it makes more sense for a biologist to answer it. My question is: what can be said, if anything, about the space of all possible human DNAs (for a suitable definition of "human" in the context of this question)? Is it open or closed with respect to a suitable topology?

EDIT: To fix ideas and make it less a matter of opinion, one (admittedly incomplete, this is for the sake of making the question more precise) definition to use might be "those who can reproduce with existing fertile human beings"?

EDIT 2: To be clear, I am not a biologist but a computer scientist and maybe my question is guided by assumptions that simply do not hold. My naive vision of someone's DNA is a string of A,T,G,C which, when "interpreted", becomes that person. I am also under the impression that a single mutation of a letter may render them nonviable. On the other hand, other mutations may produce an individual with altered characteristics, who may or may not be able to reproduce with other human beings. So I was wondering if there is an interesting way to think of the space (in a mathematical sense) of all strings of DNA which produce a viable human being in this sense. If my assumption that a single mutation can render one nonviable, I would expect that this space would be "closed" for some topology. I'm totally ready to accept that this way of thinking about DNA is flawed and/or unrealistic, and in that case I'd be happy to learn why, if possible in layman's terms.

  • $\begingroup$ The question is entirely dependent upon the definition of "human" you are willing to use. There is no obvious "suitable definition", there are only definition each person might want to come up with. I am VTC because an answer will necessarily be opinion-based. $\endgroup$ – Remi.b Apr 2 '19 at 1:03
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    $\begingroup$ I think the definition of human is less of a problem (one can probably assume we are mostly interested in healthy human cells, preferably in vivo) than the definition of open and closed topology of DNA: DNA chromatin status (which what's usually used as a proxy for the overall DNA topology) is VERY hard to measure directly and is therefore mostly assessd by indirect methods (ChiP, Acetylation state, occupancy by proteins &/ RNA-pol) $\endgroup$ – Nicolai Apr 2 '19 at 7:08
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    $\begingroup$ I hope I'm interpreting your question correctly; you are asking how large the possible conformation space is for DNA contained within 23 pairs of human chromosomes. The answer is very very large, since there are countless ways to arrange it, loop them, tangle them. Evolution has certainly sampled many ways to do this, but the number of possibilities unsampled is probably much astronomically higher. It's a bit like asking how many ways you can tangle a single, very long string, or 46 of them, for that matter, not including mitochondrial DNA. (cont...) $\endgroup$ – S Pr Apr 2 '19 at 10:57
  • $\begingroup$ (...cont) Each cell in your body is unique in this respect, although cells of similar type share similar features that may distinguish it from other cell types. Chromosome conformation is difficult to determine, and the best ways currently we have of looking at it involves looking at the modification of histones (methylation, acetylation, etc.), looking at which transregulatory elements are bound at specific locations (polymerases, cohesins that hold loops in place, which portions of DNA are accessible to DNA-cutting enzymes, etc.), and methods like chromosome conformation capture ("3C"). $\endgroup$ – S Pr Apr 2 '19 at 11:01
  • $\begingroup$ @user3078439 if you'd like a super simplified explanation of the role of DNA in "creating" a person, see here and then here for 2 quick video explanations (< 5 minutes). It's a very basic (and relatively non-biological) approach to explaining things, but the videos should confirm for you the conceptual role of DNA in a human (cont...) $\endgroup$ – theforestecologist Apr 2 '19 at 19:23