reactions that C or T undergo, or they could even have no role at all.
While we are still‘ in the dark’ about dark states, their presence represents one of the important considerations in DNA’ s relationship with UV: the amount of time the energetic excited states remain active. The longevity of the excited states is also influenced by the other structural features of DNA, namely base-stacking and base-pairing. Basestacking and base-pairing are bonding forces that hold the double helix together, and they can also be regarded as avenues for communication between nucleobases. This provides the means to either dissipate the energy of UV light on the one hand, or to spread the effects of damage on the other.
Given the complexity of DNA, the roles of base-stacking and basepairing are less well understood than the fast deactivation that occurs in single nucleobases. When the nucleobases are stacked, the excited states get longerlived, although this is quite sensitive to the type of nucleobases in each strand. There is evidence that these excited states result in the transfer of an electron from one nucleobase to another. The loss of an electron from a molecule is termed oxidation, and the promotion of anti-oxidants in our diet suggests that oxidation in our DNA is something to be avoided. Fortunately, it appears that any electron transfer that occurs after UVB exposure is efficiently reversed within about 100 picoseconds. Nevertheless, these electron transfer processes could have more subtle effects, such as making certain parts of the DNA strand more sensitive to damage than others.
Some debate also surrounds the importance of base-pairing. Computer calculations have suggested that the base-pair provides a route to rapidly deactivate the excited state. This has since been challenged by experiment, and some evidence suggests that the base-pair does not transfer energy from one strand to the next. This would have implications for the integrity of DNA, as it might prevent both strands from being damaged, thereby leaving an intact‘ copy’ of the damaged strand.
It should also be noted that basepairing does not always follow the
Before the fad of becoming browned off caught on people like Claude Monet’ s lady with a parasol preferred to stay in the shade.
‘ Watson-Crick’ protocol where A goes with T and G with C. Some nucleobases can base-pair with other identical bases, giving rise to other structures apart from the famous double-helix. For example, DNA rich in either C or G can form exotic four-stranded shapes called the i-motif and G-quadruplex, respectively. These structures have attracted attention in recent years due to their connection with telomeric DNA, the sequences in the chromosome that control cell death, and the cell‘ immortality’ associated with cancer. Both the i- motif and G-quadruplex form long-lived excited states after UV excitation, more so than in normal double-stranded DNA. Furthermore, these structures can absorb light at slighly longer wavelengths, where the effectiveness of atmospheric ozone decreases. Further study is needed to investigate how significant this is biologically, or whether damage to these rarer structures is relatively tolerable. Much remains to be learnt about the very fast but very important processes can occur in our DNA immediately after UV exposure. Despite the perceived and much lamented lack of sunshine in Ireland, Irish scientists from UCD( Dr Susan Quinn) and TCD( Prof John Kelly) have contributed significantly to this‘ exciting’ area of study. This is due to a long-running collaboration with the Rutherford Appleton Laboratories in the UK, which is home to an advanced laser set-up known as ULTRA. On ULTRA, a UV laser is shone on a piece of DNA to form the excited state, and a second infrared laser is used to examine how the bonds are altered thereafter. Any chemical changes that occur after excitation, or the movement of energy through the molecule, can then be tracked over very short timescales. Since these processes are so quick, the light from the laser pulse needs to be very short, because it is crucial to observe the DNA after the UV light has switched off. Consequently, this is one of those areas of science that is driven as much by technological progress as it is by theory or experiment. Most of the studies to date have taken a‘ bottom up’ approach, by looking at very short sequences, or sequences containing only one or two types of nucleobase. The greater challenge is to study, and more importantly, to understand, the complicated DNA sequences that exist in our cells. Of course, the eventual effects of UV light also depends on the slower biological processes in the cell, such as repair mechanisms, and the way damaged sequences are replicated. This is where the physicists and chemists step back, and the biologists and medical scientists take over. UV light is also important to biology in many other ways, besides the direct absorption by DNA. For example, a certain amount of UVB is necessary for the sythesis of Vitamin D, and the impact of lower energy UVA cannot be dismissed. UVA is not absorbed by DNA, but it is not blocked by the ozone layer either, so is up to 100 times more abundant than UVB. Its indirect effects on DNA, and consequently our wellbeing, are a serious cause for concern as well. In the meantime, it is probably prudent not to push our bodies’ hardwon defence mechanisms too far. Leave the experiments for the lab, and learn to love the shade!
Dr Paraic Keane is involved in research on photochemistry and has an MSc in Science Communications from DCU.
SCIENCE SPIN Issue 58 Page 11