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I heard a point, that all (human) body atoms are recycled withing short period like few years. Recycled means "old" atoms are replaced by "new" ones during metabolism, leaving only structure unchanged.

But this looks contradicting with knowledge about DNA. DNA molecule looks unchanged and the fate of all it's atoms looks known.

Yes I know there are some spontaneous damages which are repaired, but most of DNA atoms remain in place. Even during cell division, 50% of atoms goes to one of the descendant cells. I.e. they don't go outisde body.

So, if the point is wrong concerning DNA atoms, then may it is wrong at all?

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3 Answers 3

up vote 6 down vote accepted

Rudolf Shoenheimer and David Rittenberg were key figures in introducing the isotopic tracer technique to biology.

This technique, which may be dated to the discovery of deuterium by Urey in 1933, produced a revolution in biological thought.

Much of the early work was done at the Department of Biological Chemistry at Columbia University under the chairmanship of H. T. Clark.

Up to this time it was thought that the components of tissues were relatively stable, and that ingested food was metabolized immediately to provide a source of energy.

The first experiments showed that when deuterated fatty acids were fed to mice, most of the label was initially deposited in adipose tissue.

It was concluded that adipose tissue acts a 'fuel depot' that is in a state of metabolic flux and is the direct source of fatty acids burned as a fuel (see here for refs). Fats present in food are not primarily used an an immediate energy source.

When a stable isotope of nitrogen became available (N15), it was found that proteins were also 'in a state of dynamic flux' and were being continuously regenerated and broken down.

The first experiments were done with N15-labeled tyrosine and it it worth quoting from the paper (Studies in Protein Metabolism. VII The metabolism of tyrosine by D Rittenberg Rudolf Schoenheimer & S. Ratner)

The results indicate that in our rat the nitrogen of at least one amino acid, tyrosine, was only partly excreted in the urine, while almost half of it was retained in the body proteins.

Of this deposited nitrogen only a fraction was found attached to the original carbon chain, namely to tyrosine, while the bulk was distributed over other nitrogenous groups of the proteins

Shortly after giving a series of lectures at Harvard University in 1941, Shoenheimer took his own life. The lectures were published posthumously as a classic book called The Dynamic State of Body Constituents.

Further information about the early work on isotopes, their role in changing completely the way we think about metabolism, and the role of Columbia University in shepherding in this great era, may be found in the following open-access article by Eugene. P. Kennedy:

Hitler’s Gift and the Era of Biosynthesis

Another good reference on the work of Schoenheimer may be found here

So are all proteins broken down and regenerated? I know of any one exception and that is the lens crystallins where there is virtually no turnover throughout the life of an individual.

The crystallins in our lens are synthesized at birth and remain with us virtually unchanged for life.

There is a very nice open-access paper on this published in PLOS One:

Radiocarbon Dating of the Human Eye Lens Crystallines Reveal Proteins without Carbon Turnover throughout Life

(Lynnerup et al. 2007)

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In the original answer I said that the original isotope experiments were done with radioactive fatty acids. This, of course, is rubbish. They were done with deuterated fatty acids. – TomD Nov 1 '13 at 11:09

I think you're right. DNA contradicts the notion of "total mass turnover." So the point is not generally correct. Of course, how incorrect the statement is reflected by the mass ratio of DNA to non-DNA components -- most all of those other components do turn over multiple times in a lifespan (including things that seem frozen-out like bone calcium).

Turns out the order of magnitude estimate for the mass of DNA in a human is ( something like a pound. So the statement is on the order of 1% error for a typical adult human.

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But how do we know that other atoms are turned over? May be ribosome or cytoskeleton proteins are also persist? – Suzan Cioc Oct 30 '13 at 20:31
Are you asking, what experimental techniques monitor the turnover of biochemicals? Whole-animal metabolic studies are the main historic source of these kinds of statements. Let's say you give a mouse 14C-labeled food for its first six months of life, then switch it to unlabeled food. Monitoring the whole-mouse 14C content over time lets you quantify turnover. – Ryan Oct 30 '13 at 21:33
or even 13C - 14C is a radioactive isotope :) expensive! – shigeta Oct 31 '13 at 13:17
still with cell turnover and early growth, we're talking about just a few percent of the cells whose DNA is not new every few years. – shigeta Mar 17 '14 at 14:26

Y'all are neglecting the fact that cells die and are continually replaced, which would include replicating the DNA with new atoms. The most interesting study I've seen on this is summarized in this New York Times article. Above ground nuclear testing up until 1963 released a great deal of Carbon-14 into the atmosphere. This ultimately became incorporated into the DNA of anyone living at the time, and was used as a marker for how quickly specific tissues turned over in the body.

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