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Calf-thymus DNA is widely used as DNA sample. Such as testing anti-dsDNA antibody activity, nuclease activity etc, as well as certain books include calf-thymus-DNA in various examples (such as the book Molecular biology by Weaver (ed-5) mentions in a table on % (G+C), that %(G+C) of calf-thymus DNA is 40)

Now; my question is; why a peculiar organ like thymus is selected (and not liver or kidney etc); and not any organ from adult cows (which are more available at any meat shop)? whereas all the otherly-situations should contain the same DNA-sequence? Isn't it a bit weird?

Is it more easy to extract the DNA from a calves' thymus gland?

I've searched web, but nothing helpful I found.

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    $\begingroup$ The thymus gland is much larger in prepubescent calfs (and humans) than in adult cows, so more DNA can be extracted from it. $\endgroup$
    – user30455
    Mar 5 '17 at 21:03
  • $\begingroup$ @user30455 but why not cow's brain or heart or veins etc... such "common organs" (though all of them should theoretically same DNA). That is the main point of this question. Your point is correct... to get the thymus-DNA, adult cow is not right source... but that is not the question here. Here the question is why thymus gland for DNA? $\endgroup$
    – Always Confused
    Mar 10 '17 at 16:52
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I have crawled through google and many, many journal articles. What I can make of it, is:

"It is generally agreed that from mammalian cells (as, for example, calf thymus..." are capable of yielding high molecular weight DNA from cells, with very little protein present to decrease purity. [From Welsh and Vyska]

Additionally: "Calf thymus gland is a fairly good source of replicatiue deoxynucleotidyl transferase (deoxyribonucleic acid polymerase)" [From Yoneda and Bollum]

So we have a good DNA coming from the calf thymus. Calf thymus is also something we have in great abundance (worldwide slaughter of cattle) and I don't know of any other demand for it (don't know of anyone eating it?). So it should be effective and not too expensive, relatively.

Perhaps there is a difference between the thymuses (thymi?) of calves and adults, given the immune role of the thymus, it's possible. But I can only speculate on that.

And yes. It does seem a bit weird. But it works and everybody does it, so we seem stuck with it.

My two sources:

Richard S. WELSH / Karel VYSKA, Relationship between the Purity and Molecular Weight of Calf Thymus DNA, Hoppe-Seyler's Z. Physiol. Chem. Bd. 362, S. 969-981, Juli 1981

M. YONEDA AND F. J. BOLLUM, Deoxynucleotide-polymerizing Enzymes of Calf Thymus Gland, THE JOURNAL OF BIOLOGICAL CHEMISTRY, Vol. 240, No. 8, August 1965

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  • $\begingroup$ Thymus gland tend to deteriorate with age, so I didn't asked "why not adult cow thymus DNA; rather I asked "why not adult cow organs' DNA". I meant adult-cows are more used for meat purpose so far I know... so people would look for any organ of adult cow- not of baby cow. And kidney etc are not ate so far I know... may be I don't know its details. $\endgroup$
    – Always Confused
    Nov 6 '16 at 18:52
  • $\begingroup$ "we seem stuck with it" means its jut tradition? is that? $\endgroup$
    – Always Confused
    Nov 7 '16 at 4:25
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    $\begingroup$ @AlwaysConfused and yes, "we seem stuck with it" basically means it's tradition. I more mean that it is accepted practice, in that it is the consensus of the scientific community "this is how you do this". $\endgroup$
    – GrumpyMammoth
    Nov 7 '16 at 5:45
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    $\begingroup$ So when it came time to identify a source of carrier DNA, calf thymus (obtained free from a slaughterhouse, as described above) was one convenient source. Salmon sperm, was also popular, and salmon sperm DNA was eventually sold by Sigma-Aldrich. My postdoctoral mentor, Bill Wood, who was a graduate student at Stanford with Paul Berg (he started the year Kornberg was awarded the Nobel Prize) tells of dressing in a suit, and carrying an ice bucket to go collect calf thymus at an abattoir. The worker offered him some to take home for cooking (sweetbreads). $\endgroup$
    – mdperry
    Nov 7 '16 at 13:32
  • $\begingroup$ excluding chromosomal proteins; all cells of the organism should contain same-molecular weight DNA. Maybe certain organ contain more amount or copies of DNA; but since their sequence is same; no-one will be higher in molecular wt. $\endgroup$
    – Always Confused
    Nov 7 '16 at 20:05
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I am afraid if speculation based answers are tolerated here, but I assume DNA extraction from certain organs might were more easier than the other.

At a period when any kind of PCR or in-vitro replication methods were not available, people required to isolate bulk amount of DNA.

Now observe, 3 earliest natural sources from where DNA were isolated; have a difference from other cells.

  1. Salmon sperm DNA : It did not contained histone. Instead it contained protamine. Also loose cells, lower extracellular debris.

  2. Mischer's nuclein from pus cell: White blood corpuscles have enlarged nucleus and likely a high transcription and translation activity. A high transcriptional activity is likely create unwined portions of DNA that lack histones. The transcription and translation activity might be even higher when the WBCs are in fight with microbes because they will require to synthesize a lot of antibodies and lysosomal enzymes. Also loose cells, not much extracellular debris.

  3. Thymus gland : a site of cell division and maturation, i.e. high replication and transcriptioon activities. Also a spongelike tissue, not very adherent nor stiff extracellular matter.

It seems these factors were helpful to easily separate nucleic acids from random biomolecules such as proteins and polysachharides.

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