Finding supports model on cause of DNA's right-handed double helix

Finding supports model on cause of DNA's right-handed double helix
A drawing by University of Nebraska-Lincoln physicist Timothy J. Gay illustrating how electrons in cosmic rays, which have mostly left-handed spins, could have bombarded pools of ooze on the surface of the primordial Earth, preferentially destroying left-handed precursors of DNA, leaving the right-handed-only DNA that exists today.

The DNA of every organism on Earth is a right-handed double helix, but why that would be has puzzled scientists since not long after Francis Crick and James Watson announced the discovery of DNA's double-helical structure in 1953.

It's a puzzle because no one has been able to think of a fundamental reason why DNA couldn't also be left-handed.

New research by University of Nebraska-Lincoln physicists and published in the Sept. 12 online edition of Physical Review Letters now gives support to a long-posited but never-proven hypothesis that in cosmic rays—which are mostly left-handed—preferentially destroyed left-handed precursors of DNA on the primordial Earth.

The hypothesis, called the Vester-Ulbricht model, was proposed by Frederic Vester of the University of Saarbrucken in Germany and Tilo L.V. Ulbricht of the University of Cambridge in England in 1961 in response to the 1957 discovery that most of the electrons spewing from radioactive beta decay were left-handed.

Joan M. Dreiling and Timothy J. Gay of UNL focused circularly polarized laser light on a specially prepared crystal of gallium-arsenide to produce electrons whose spins were either parallel or anti-parallel to their direction of motion upon emission from the crystal—essentially artificial beta rays. They then directed these electrons to strike target molecules of a substance called bromocamphor, which comes in both right- and left-handed varieties.

They found that at the lowest electron energies they studied, left-handed electrons preferentially destroyed left-handed molecules and vice versa. This sensitivity to molecular handedness has a mechanical analog: the inability of a left-handed bolt to screw into a right-handed nut. The molecular experiment proves the principle underlying the Vester-Ulbricht hypothesis.

Finding supports model on cause of DNA's right-handed double helix
University of Nebraska-Lincoln physicists Joan M. Dreiling and Timothy J. Gay with a sculpture of a DNA molecule on display in UNL's Beadle Center for Biotechnology. Their experiment proved the principle underlying the Vester-Ulbricht hypothesis that the primarily left-handed spinning electrons in cosmic rays could have preferentially destroyed left-handed precursors of DNA, leaving only right-handed DNA. The sculpture illustrates DNA's right-handed double helix.

"The circular polarization of the laser light effectively transferred to the spin (handedness) of the electrons emitted by the gallium-arsenide crystal," said Dreiling, a postdoctoral research assistant who received her doctorate from UNL in May. "We are able to reverse the spin-polarization of the electrons just by reversing the of the light."

The effect they saw was quite small, they said—like "looking for an electronic needle in a haystack," Gay said—but they said they're highly confident in their result.

"We have done several different checks with our experiment and I am totally confident that the asymmetry exists," Dreiling said. "The checks all came out showing that this asymmetry is real."

Gay, a professor of physics and astronomy, said the paper in Physical Review Letters culminates a 21-year effort that began in earnest when he came to UNL from the University of Missouri-Rolla in 1993.

"This has been an incredibly hard experiment," he said. "I've ground two graduate students in the dust. Poor Joan survived. The others got their Ph.D.s in other things and a lot of good science came out on the way, but Joan was clever enough to make this experiment work.

"What she did was make the first experiment that showed the asymmetry at the molecular, nano level. That's the molecular physics part of it, which is what we're really interested in, but there's also this tie to the origins of life on Earth."


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More information: Chirally Sensitive Electron-Induced Molecular Breakup and the Vester-Ulbricht Hypothesis, journals.aps.org/prl/abstract/ … ysRevLett.113.118103
Journal information: Physical Review Letters

Citation: Finding supports model on cause of DNA's right-handed double helix (2014, September 18) retrieved 14 October 2019 from https://phys.org/news/2014-09-dna-right-handed-helix.html
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Sep 18, 2014
"I've ground two graduate students in the dust. Poor Joan survived. The others got their Ph.D.s in other things and a lot of good science came out on the way, but Joan was clever enough to make this experiment work.

Dammit. That's one big risk to take during the job interview: "The two guys before you failed. Are you willing to risk 3-5 years where you may not get your PhD at the end?"
Kudos to her for seeing it through!

Sep 18, 2014
There are many ways to break symmetry and get chirality in polymers. This doesn't predict why evolution chose left-handed amino acids, who were present in the early oceans from abiotic source production.

More likely then, the winning chirality was a product of evolution where some early system happened to be more fit at the time.

@ogg_ogg: There is an embarrassing _multitude_ of plausible pathways for emergence of life. That isn't the problem, the problem is to test them.

Fortunately phylogenies stretch that far today, due to discovered homologies between Hadean geophysical and cellular systems. Evolution for the win, it has now conquered emergence of life as well!

The most plausible pathway today would be alkaline submarine hydrothermal emergence, now with 8 or so homologies with later cells. But there are yet tests to be done, and a consensus to form. It may take decades before this question is folded.

Sep 18, 2014
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Sep 19, 2014
@Goika: Why would (racemic polymer) micelles break chirality, preferentially absorbing one chirality over another (depending on curvature?)?

Also, this hypothesis of yours is an RNA world one, so it won't fit metabolic hypotheses say. If we look at them, we get a generic mechanism for chirality breaking. E.g. evolution on racemic competing pathways would randomly pick one over another - but pick it must. This is also how most chemical chirality breakings happens, a small difference gets amplified.

[This is my takeaway from chemical chirality breakings btw, YMMV:

I don't find the search for extraneous environmental mechanisms as described in the article useful. They are answers looking for a non-existent problem, the breaking is inherent in competition in a finite environment.]

Sep 19, 2014
It is interesting to note how these hypotheses are rooted, by the way.

They arise because of weak, potentially too weak, forcing of radiative influx on an RNA world "soup" environment. Thermal and UV fluxes is the only forcing for the necessary disequilibrium chemistry that evolves from soup to cell. That would be forcing on the order of 0.1 eV (thermal) and 1.0 eV (UV), the later can not only produce disequilibriums but also rearrange chemical bonds.

This does not lead to chemical competition as much as disarray, fragmentation of resources. (Woese.)

Instead redox and pH chemistry forcing on the same magnitude and direction as today's cells as happen in a vent "battery" environment, 2 eV, would not only rearrange bonds but drive a whole metabolic pathway. (With the help of metabolic engines as organizers, naturally, such as Mn or W electron bifurcating atoms.)

Closing the loop on efficient production enforces chirality breaking by competition for resources.

Sep 19, 2014
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Sep 20, 2014
There are many ways to break symmetry and get chirality in polymers. This doesn't predict why evolution chose left-handed amino acids, who were present in the early oceans from abiotic source production.
Ummm, did you forget the handedness of DNA just mentioned in this article, which would then select for the handedness of the amino acids, no matter what had happened before? Using DNA obviously conveys an evolutionary advantage. All the creatures that used right-handed amino acids couldn't use DNA, because it was the wrong chirality for their amino acids; and DNA was so successful they all died out.

Seems to me you're putting the cart before the horse.

Sep 20, 2014
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Sep 20, 2014
@Goika/Eseta: So you have no evidence for that non-chiral micelles breaks symmetry?

I'm not sure what your claim is then. That already chiral RNA makes micelles that further breaks chirality (in amino acids and sugars) perhaps? Interesting, but not the question at hand. Also, the chiral pentose sugars are part of RNA, so there would no longer be any chiral break after nucleotides were chosen.

"In my model the RNA served originally as the digesting enzymes".

This again depend on the question above, how RNA became chiral. If it wasn't before arriving at enzyme activity, your theory is a metabolic one. If it was, presumably your theory derives from an RNA world replicator mechanism.

Sep 20, 2014
@Da Schneib: DNA wasn't involved in the selection of amino acid handedness, it happened in RNA/protein cells (as Goika notes) according to ribosome phylogenies. There was a Nobel prize (-98, I think) when that was confirmed (only RNA in the core protein making machinery).

And today people can see details in how the ribosome went from producing dipeptide cofactors on ancestral tRNA (which had enzymatic function in the anoxic, Fe-II rich Hadean Earth) to polypeptide metal co-factor binding "nests" to eventually translating mRNA.

And that is my point there, there were several breaks of chirality (nucleotides/sugars, amino acids), and predicting one of them do little for the other.

Worse, since they have different handedness no single mechanism suffice. Instead, it suggests a random outcome based on evolution.

Sep 20, 2014
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Sep 20, 2014
@Da Schneib The DNA evolved much later than the aminoacids based life. If nothing else, RNA was here first with asexual prokaryota. So that the handedness of DNA cannot explain the handedness of aminoacids. Who is putting the cart before the horse?
But the only RNA and amino acids that survived are the kind that work with the kind of DNA that isn't destroyed by the electrons and positrons. Since this effect acts on only one kind of DNA, RNA organisms whose RNA and amino acids weren't compatible with the surviving type of DNA were outcompeted and died out.

That's how evolution works.

Torbjorn, that answers you as well.

RNA-based creatures still exist, IIRC, but not very many and only in niche environments that select against DNA. One of them, IIRC, even uses a different RNA nucleotide, unique (unless someone has discovered another one I haven't heard about) among all life on Earth.

Sep 20, 2014
I looked in Wikipedia and it seems that other nucleotides are fairly common in RNA, but as far as I could tell not in coding RNA. However, I couldn't find the article I was looking for on the RNA-based creature (single-celled IIRC) that uses a different codon, so I can't be sure others haven't been discovered, or a whole class even. I'm not done looking though.

Also Torbjorn, evolution is not random. It is directed by natural selection, which is not a random process, despite being driven sometimes by random physical events.

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