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Books always refer to human DNA as a unique molecule, and two different humans generally have different DNA molecules. But how many different types of DNA molecule does a person have?

In order to make this more understandable, let's give a physical/mathematical definition of molecule equivalence classes. Two molecules are in the same equivalence class if they have the same atoms in the same relative position to each other. And we say these two molecules are equivalent. In other word they are chemical clones of each other, like two $H_2O$ molecules are equivalent.

My question is this: how many equivalence classes of DNA does one human have (before being damaged or mutated)? 1, 23, 46, 92 or another number? Does a human have 46 or 92 different kinds of DNA molecules or just one that wraps differently to give different chromosomes? The same question could be given for a whole person or a single human cell.

Yes, I have looked on the internet and even asked a biologist. It seems the answers are always confusing to me because of the definition of the terms. What biologists seem to call "the DNA molecule" seems to be an ensemble of different equivalence classes of molecules, and I am finding this confusing.

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    $\begingroup$ It's worth doing some research before posting here. A fine start might be to look up "chromosome" or "human chromosomes". $\endgroup$ Sep 30, 2021 at 21:29
  • $\begingroup$ Despite your interchange with @jakebeal perhaps persuading you that you are "right", you should try to appreciate why your question has been so badly received. We don't understand you. Not because we have no mathematics, but because you are talking in terms that do not relate to what we know and how we think about chromosomes. So even with your explanation, equivalence class means little to me. If we are talking about molecules of DNA, presumably one molecule is one chromosome. So, to us, your question becomes "how many chromosomes?" which you can find from the Internet. So what do you mean? $\endgroup$
    – David
    Oct 11, 2021 at 18:20
  • $\begingroup$ @David one same molecule could wrap differently to produce the different chromosomes. I talked with biologists who believed this was the correct answer, admitting it was confusing. I don't believe it is the correct answer, it seems unrealistic. So far I believe the correct answer is that humans have 23 or 46 or 92 different DNA molecules. I don't care about being downvoted. I don't care about points. I just want an answer and help others if I can. Thank you $\endgroup$
    – ceillac
    Oct 29, 2021 at 15:54
  • $\begingroup$ And I was trying to help you, not by telling you how to get points, but how to get an answer. And no, different chromosomes are not formed by different conformations of the same DNA. This has been known for ever because of their different sizes and appearances, but they have all been sequenced now. And if you are talking in multiples of 23, I assume you are entering the semantic wilderness of whether one strand or two strands of DNA constitutes a molecule. You can find that fruitless question has already been asked and the answer is yes. $\endgroup$
    – David
    Oct 29, 2021 at 17:05

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It seems to me that you are becoming confused because you are thinking of DNA in the same way that one things about small molecules.

When molecules are enormous, however, they act more like a material than a simple molecule and have no single structure. A good starting place for learning how to think about DNA as a large molecule is to learn about polymer chemistry.

Furthermore, a critical difference between these molecules and prototypical small molecules like water or glucose is that the small molecules are largely rigid in their structure. Biopolymers like DNA, RNA, and proteins (not to mention complex carbohydrates and lipid structures), on the other hand, are flexible. This means that the number of geometric configurations of such molecules is infinite.

This flexibility is critical for life, since it allows the molecules to retain their integrity even while interacting with other molecules to perform biological processes. That extends even to modification of the molecule itself, e.g., via epigenetic modification. In biology, we often talk about this as though the molecule is the same (again, treating it more like a material rather than a small molecule), even though technically its chemical species has indeed changed. With such an unbounded number of potential variant species, however, this sort of shorthand makes pragmatic sense for discussion, even if it does not match the definitions from Chemistry 101.

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  • $\begingroup$ sorry to think of molecules as ... molecules. Thank you so much for your wisdom pointing out exactly my problem. $\endgroup$
    – ceillac
    Oct 2, 2021 at 22:54
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    $\begingroup$ @ceillac It's an easy mistake to make, since early chemistry classes generally focus on either small molecules or crystalline ionic solids, both of which are far more well-behaved and easy to understand than the weirdness of enormous wibbly-wobbly biopolymers. $\endgroup$
    – jakebeal
    Oct 3, 2021 at 19:53
  • $\begingroup$ I was being ironic. This site has become a parody. I am out $\endgroup$
    – ceillac
    Oct 4, 2021 at 20:27
  • $\begingroup$ @ceillac Would you care to point out what I am misunderstanding in your question? I'm happy to attempt to amend or clarify. $\endgroup$
    – jakebeal
    Oct 4, 2021 at 22:51

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