N.Wells
Posts: 1836
Joined: Oct. 2005
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I got side-tracked before completing a previous post.
From the discussion at
https://www.researchgate.net/post......._of_DNA
Note in particular:
From Richard J Edwards · University of New South Wales
| Quote | | Technically, dsDNA would be two molecules. During replication, there could be thousands, until DNA ligase has done it's job. I guess anything from a dinucleotide upwards would count as a DNA molecule, as DNA is a polymer. |
From Max Robinson · Institute for Systems Biology
| Quote | | I agree with Jon ahd Richard (and Wikipedia), a molecule is defined by covalent bonds; a(n undamaged) double-stranded DNA chromosome is really a complex of two single-stranded molecules held together by a variety of non-covalent bonds and forces, and decorated with a plethora of proteins that form chromatin. As Richard points out, one chromosome undergoing replication typically contains many DNA molecules, with the leading strands replicated in longer molecules and the lagging strand replicated in shorter molecules that are eventually ligated together. |
From Charles A Miller · Tulane University
| Quote | | I consider each single DNA strand to be a molecule. Including hydrogen bonds and other non-covalent interactions in a "definition of a molecule" would not only give you double stranded DNA, but would include all the proteins that constitute chromatin. The single strand of DNA is also the functional unit as DNA is replicated or transcribed by polymerases that "read" a single strand. Consider a water molecule. The water molecule is hydrogen bonded to other water molecules, but we don't consider the volume of water and all the molecules it contains to be the molecule. |
From Gurbachan Miglani · Punjab Agricultural University
| Quote | | Single-stranded DNA, since it is covalently bound, is one molecule. Double-stranded DNA has two molecules connected through hydrogen bonds and hydrophobic interactions. |
From Giovanni Cercignani · Università di Pisa
| Quote | | My opinion is that you must first state if you are speaking of DNA as a chemical compound or as a biological object, because the two are conceptually different. As pointed out by many commenters, a chromosome (or a chromatid) may happen to get some accident during its "life" (i.e. between two replication events). So, the functional (biological) point of view may help to get a provisional answer, that is conceptually operative. Irrespective of its actual dymanic/metabolic state, a DNA double/single stranded molecule (depending on the genome structure) contains a set of instructions (= coded information) which can operate, by the assistance of external tools such as proteins and other biomolecules, to transmit/express the information. The point with DNA a a single functional entity is that all those instructions typically function as "cis-acting elements", as genetists call them; instructions are meaningful for sequences on the very same DNA molecule (some of them are meaningful only for the double-stranded entity - that's why I think a rigorous chemist's approach will fail to help). If you skip a few complex mechanisms that escape from the above statement of cis-acting elements, this can give you a definition of "single DNA molecule" in a living cell. You know, there are no universal rules in biology, you can always find exceptions. |
So the consensus seems to be that one strand of DNA is one molecule and that two strands joined in a double helix are two joined molecules, but to get to that condition a great many molecules have been involved, many being assembled into one macromolecule and many others assisting along the way.
This contradicts your usage of unimolecular "as opposed to a number of molecules working together as a system".
Thus (my conclusion, with me not being a chemist) calling the end result unimolecular seems misleading at best.
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