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This seems to be a simple question but I have failed to find an answer by searching.

Let's consider human DNA so 23 chromosomes.

The first question is on the double helix, is that considered as one or two molecules? This is the less interesting question to me as I know the structure and it is mainly a matter of the definition of the term molecule.

The bit that is more interesting to me and I cannot find a clear answer on is the chromosomes. Is each chromosome a separate molecule or pair of molecules (hence 23 or 46) or is the whole of the DNA a single molecule or pair (hence 1 or 2). If each chromosome is not a separate molecule then how are they connected? If the whole DNA is a single molecule or a pair then what happens at the chromosome boundary?

I am trying the chemistry stack exchange as I am just interested in the chemical structure and not its biological function.

A clarification: I am not suggesting that a chromosome consists solely of DNA. I mean the DNA within the chromosome.

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When you take a single somatic cell that is not replicating, lyse it, remove the membrane lipids, remove all the protein, you would be left with just nucleic acids.

If each DNA strand were intact, and none of them are covalently cross-linked, you would expect 23 x 4 molecules.

However, DNA undergoes single-strand breaks. They can be repaired fairly easily when the second strand is intact. However, there might be thousands of single-strand breaks at steady state, so thousands of molecules.

Inter-strand crosslinks are less common but possible. For each of those between two as-of-yet unconnected strands, you reduce the number of molecules by one.

So if you take a chemical view, the number of molecules of DNA is largely undefined. If you take a biological view, you would just simply say that humans have 23 chromosomes, and learn about what they do in various stages of the cell cycle.

Is each chromosome a separate molecule or pair of molecules (hence 23 or 46) or is the whole of the DNA a single molecule or pair (hence 1 or 2).

I don't think anyone would have a good argument for describing the DNA in a human cell as a single molecule, or two.

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  • $\begingroup$ Why 23 x 4 and not 23 x 2? $\endgroup$
    – badjohn
    Nov 14, 2023 at 22:40
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    $\begingroup$ Mom's and dad's copy (pairs of homologous chromosomes, except for X and Y in males), and each is a double helix. $\endgroup$
    – Karsten
    Nov 14, 2023 at 23:10
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Is each chromosome a separate molecule or pair of molecules (hence 23 or 46) or is the whole of the DNA a single molecule or pair (hence 1 or 2).

None of the options.

Chromosomes are organelles, consisting of a DNA molecule (depending on a view, a single molecule or the complementing two ones, bound by multiple hydrogen bonds), wrapped in various histone and chaperone proteins. By other words, they are much more complex than a virus, which is not a molecule either.

Human genome consists from 22 chromosome (organelle) pairs plus chromosomes X and Y, so 46 wrapped parts of DNA as dual helixes. Each pair represents two variants of the (almost) same information.

Additionally, there are multiple short mitochondrial DNA segments in typically 1000 mitochondrions. BTW, scientists among Star Wars fans must have noticed similarity between a mitochondrion and a midichlorian.

More in Wikipedia.

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Well the two strands together make the DNA molecule. you have 46 chromosomes that half of them are taken from your biological father and the other half from your biological mother so you have 2 copies of each chromosome that are like two exact houses but with different people inside so the two copies have different genes. you can not see your chromosomes unless your cells are duplicating that's when chromosomes start to form. chromosomes are completely compressed in metaphase which you can see them in a karyotype. in a normal cell chromosomes are not visible because the whole sequence is tangled together.

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