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As I was reading Griffith's Introduction to genetic analysis this evidence was provided for single DNA makes single chromosome.

Eventually geneticists demonstrated directly that certain chromosomes contain single DNA molecules. One strategy was to use pulsed field gel electrophoresis, a specialized electrophoretic technique for separating very long DNA molecules by size. The extracted DNA of an organism with relatively small chromosomes, such as the fungus Neurospora, is subjected to electrophoresis for long periods of time in this apparatus. The number of bands that appear on the gel is equal to the number of chromosomes (seven, for Neurospora).

(Page 86).

This points to the fact that one chromosome has single DNA. What I want to know is that what evidence is there that each chromosome is not made up of multiple similar sized DNA molecules all coiled together to give a single chromosome so that the number of different types of DNA molecules to number of chromosomes have ratio one to one but one type of DNA molecule's multiple copies makes one chromosome.

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  • $\begingroup$ I don't have a copy of Griffith, so I can't check myself. But are you sure that the experiment he describes is not to demonstrate that each Neurospora chromosome is made of one dsDNA molecule (two strands, incidentally) of length 10 arbitrary units, say, rather than three different dsDNA molecules of length 1 + 3 + 6, say. The question you ask is quite valid, but, I imagine, addressed in earlier in other ways, especially in bacteria. $\endgroup$ – David May 25 at 16:41
  • $\begingroup$ I think that this and along with some other experiments given in Griffith tries to proves both of these points. $\endgroup$ – Arnab Ray May 25 at 20:25
  • $\begingroup$ I doubt it. I don’t know the answer for certain, but it seems to me that electron microscopy of bacterial chromosomes (no complications of chromosomal proteins) with size markers might have been a possible approach. Pulse field gel electrophoresis separates chromosomes by size but I don’t think it answers your question. I’d offer to research it for you, but I have other fish to fry at the moment. $\endgroup$ – David May 25 at 20:57
  • $\begingroup$ What do you mean by "coiled together"? I guess you mean kept together by some non-DNA material, i.e. proteins. Proteins are removed using proteinase k or something similar, so this would break any protein fostered coiling between the molecules and you will see a crazily small chromosome. You can find an example of this kind of experiments ehre (scielo.br/…) and some reading and exercises here: ncbi.nlm.nih.gov/books/NBK21330 $\endgroup$ – Fabio Marroni May 29 at 13:51

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