New Look At DNA Hints At Origin Of Ultraviolet Damage

August 25, 2005

Chemists at Ohio State University have gained new insight into how sunlight affects DNA. And what they found overturns ideas about genetic mutation that originated decades ago.

In the current issue of the journal Nature, Bern Kohler and his colleagues report that DNA dissipates the energy from ultraviolet (UV) radiation in a kind of energy wave that travels up the edge of the DNA molecule, as if the energy were climbing one side of the helical DNA "ladder."

The finding lends insight into how DNA damage occurs along the ladder's edge.

It also counters what scientists proposed in the 1960s: that UV causes mutations by damaging the bonds between base pairs – the horizontal "rungs" on the ladder. The new study shows that UV energy moves vertically, between successive bases.

In undamaged DNA, there are no chemical bonds between vertically stacked bases. But the bases do interact electronically, which is why Kohler thinks they form an efficient conduit for UV energy to flow through.

"Even though paired bases are connected by weak chemical bonds, it's the interactions that take place without chemical bonds – the interactions between stacked bases – that are much more important for dissipating UV energy," Kohler said.

The Nature paper builds on work from five years ago, when the associate professor of chemistry and his team first discovered that single DNA bases convert harmful UV energy to heat to prevent sun damage in the same way that sunscreen molecules protect sunbathers.

Back then, they studied only single bases floating in water. They hit the bases with a kind of UV strobe light, and saw that the energy was released as heat in less than one trillionth of a second.

Their new experiments show that the behavior of full DNA differs profoundly from that of isolated bases. When the chemists turned their strobe light on whole strands of novel DNA, the UV energy still changed to heat eventually, but the energy dissipated a thousand times more slowly.

That's an eternity in the DNA universe, where scientists need to use special equipment just to see these ultra-fast chemical reactions happen. Yet, Kohler's team saw no evidence that the UV affected the chemical bonds between the base pairs. They surmised that the UV energy was leaving the molecule by traveling along the edges instead.

"This slow relaxation of energy is utterly different from the mechanism in single bases that transforms the energy into heat in less than a trillionth of a second," Kohler said.

"Eventually, the energy does turn into heat, but the important point is that the energy is retained within the molecule for much longer times," he added. "This can cause all kinds of photochemical havoc."

It could be that when base pairs are aligned in their natural state in a DNA strand, the electronic interactions along the stack provide an easier way for DNA to rid itself of UV energy, compared to passing the energy back and forth between the two bases in a base pair as scientists have previously thought.

In fact, it was the brilliance of James Watson and Francis Crick's discovery of the structure of DNA that kept this secret hidden for so many years, Kohler said. Their work revealed that the DNA helix was composed of paired bases, and that discovery led researchers to focus on how UV energy might interact with base pairs.

"In fact, so much attention has been paid to base pairing that this other interaction, base stacking, has been neglected," Kohler said.

Base stacking is frequently overlooked, he admitted, because the ladder terminology that we use to describe DNA structure makes us think that there are open spaces between successive rungs of base pairs.

A better analogy would be a stack of coins, he said. Bases are stacked right on top of each other.

Here's what he and his team suspect is happening during the UV energy wave: as sunlight warms our skin, UV photons are absorbed by the bases, causing their electrons to vibrate. These high-energy vibrations nudge the atoms in the bases around, but only along one edge of the DNA ladder at a time.

If all goes well, the DNA returns to normal after the energy wave passes. But some of the time, the atoms don't return to their original positions, and new chemical bonds are formed.

Scientists know that such accidental bonds create "photolesions" – injuries that prevent DNA from replicating properly. The details of the process aren't fully understood, but studies suggest that photolesions cause genetic mutations that lead to diseases such as cancer.

This new research helps explain why most photolesions are formed between bases on the same side of the DNA strand.

Scientists believe that proteins in the body repair DNA by removing photolesions and filling in new material, using the remaining DNA strand as a template.

If UV damage is confined to one side of the DNA double helix or the other, then the undamaged side makes an easy template for the proteins to follow. But if both sides of a strand were damaged, then the template would effectively be missing.

The Nature paper highlights the shortcomings of previous studies, which applied results from isolated base pairs to the full DNA molecule. "It turns out that you can't extrapolate the results of base pairs to whole strands of DNA," Kohler said.

The discovery has clear implications for biology, since it can help explain the DNA repair process.

"The ability to observe what happens to electronic energy in DNA on such short time scales also extends the hope that methods such as ours can finally determine how DNA is damaged by UV light in the first place," Kohler said.

The Ohio State chemists are now testing other DNA strands. For simplicity, they first wanted to compare the behavior of single Adenine and Thymine bases with strands entirely composed of those two bases. That is the work described in Nature.

Kohler's team includes Carlos E. Crespo-Hernández and Boiko Cohen, both postdoctoral researchers in the Department of Chemistry at Ohio State. Their work was sponsored by the National Institutes of Health and the Alexander von Humboldt Foundation, and was performed at Ohio State's Center for Chemical and Biophysical Dynamics, which is funded by the National Science Foundation.

Copyright 2005 by Space Daily, Distributed United Press International

Explore further: From fluids to flames, research on the space station is helping advance technology

Related Stories

Clamshell-shaped protein puts the 'jump' in 'jumping genes'

August 19, 2015

Scientists at Johns Hopkins report they have deciphered the structure and unusual shape of a bacterial protein that prepares segments of DNA for the insertion of so-called jumping genes. The clamshell shape, they say, has ...

Flexible, biodegradable device can generate power from touch

August 12, 2015

Long-standing concerns about portable electronics include the devices' short battery life and their contribution to e-waste. One group of scientists is now working on a way to address both of these seeming unrelated issues ...

Researchers use 'seafloor gardens' to switch on light bulb

August 6, 2015

One of the key necessities for life on our planet is electricity. That's not to say that life requires a plug and socket, but everything from shrubs to ants to people harnesses energy via the transfer of electrons—the basis ...

Computing at full capacity

August 2, 2015

Over 12 million servers in 3 million data centers in the U.S. burn about 100 billion kilowatt-hours of electricity every year. Billions of dollars are spent on data center energy every year, with billions more spent on power ...

Plant light sensors came from ancient algae

July 28, 2015

The light-sensing molecules that tell plants whether to germinate, when to flower and which direction to grow were inherited millions of years ago from ancient algae, finds a new study from Duke University.

Recommended for you

New Horizons team selects potential Kuiper Belt flyby target

August 29, 2015

NASA has selected the potential next destination for the New Horizons mission to visit after its historic July 14 flyby of the Pluto system. The destination is a small Kuiper Belt object (KBO) known as 2014 MU69 that orbits ...

Interactive tool lifts veil on the cost of nuclear energy

August 24, 2015

Despite the ever-changing landscape of energy economics, subject to the influence of new technologies and geopolitics, a new tool promises to root discussions about the cost of nuclear energy in hard evidence rather than ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.