How Light Effects DNA

Photo: Beaming sunlight in cloud-free sky

Diagnose genes with light - the
different reaction of DNA to light
might make it possible one day;

DNA is highly resistant against alteration by ultraviolet light, but understanding the mechanism for its photostability presents some puzzling problems. A key aspect is the interaction between the four chemical bases that make up the DNA molecule.

It has been known for years that the individual bases that code the genetic information contained in DNA show a high degree of photostability, as the energy that they take up from UV radiation is immediately released again. Surprisingly, however, it was found that in DNA which consists of many bases, those mechanisms are ineffective or only partially effective. It seemed that the deactivation of UV-excited DNA molecules must instead occur by some completely different mechanisms specific to DNA. Through measurements by a variety of methods on DNA molecules with different base sequences, a research group led by Friedrich Temps has now been able to confirm and clarify that assumption.

According to Temps, "DNA achieves its high degree of photostability through its complex double-helix structure. The interactions between bases that are stacked one above another within a DNA strand, and the hydrogen bonds between the base pairs of the two complementary single strands in the double-helix play key roles. Through the different interactions that we have observed the DNA acts to some extent as its own sun-protection".

Nina Schwalb investigated many different base combinations in synthetically-produced DNA molecules. Using a femtosecond pulsed laser spectroscope, she measured the characteristic energy release for each combination. She was able to measure the time for which the molecules continued to fluoresce, and thus how long they stored the light energy. She found that for some base combinations these fluorescence ‘lifetimes’ were only about 100 femtoseconds, whereas for others they were up to a thousand times longer. A femtosecond is one millionth of a billionth of a second.

Concluding from her research, Schwalb says: “We have found that different base combinations have widely different fluorescence lifetimes. This could lead to the development of a new diagnostic method whereby laser light could be used to directly recognise certain genetic sequences".; Source: Christian-Albrechts-Universität zu Kiel