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