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Light-induced reactions of Escherichia coli DNA photolyase monitored by Fourier transform infrared spectroscopy. FEBS Journal. Interactions between yeast photolyase and nucleotide excision repair proteins in Saccharomyces cerevisiae and Escherichia coli. When this happens, the excited molecule can excite an adjacent oxygen atom, turning the stable molecule into a reactive species. Oxygen is much less stable in its excited, higher energy state, so it will react with any proteins or lipids it collides with in the cell in order to go back to its more stable, lower energy state.
Although it can damage various molecules in the cell, the most damage occurs when it hits DNA. When an excited oxygen hits DNA, it can cause a guanine to thymine transversion, which means that the purine guanine is replaced by the pyrimidine thymine.
As in the case of direct DNA damage, this mutation alters how the DNA is translated into a protein and can be potentially harmful. Part of what makes this type of DNA damage particularly dangerous is that it is caused by excited oxygen molecules, not the UV light itself.
Excited oxygen has an unusually long lifespan for a reactive species, so the damage can occur in cells other than skin cells. Damage can also arise if the excited oxygen collides with a molecule of hydrogen peroxide - the same compound in household disinfectant. Hydrogen peroxide is produced in the mitochondria as a by-product of cellular respiration.
The cell usually turns the peroxide into water, but some molecules escape this process. If an excited oxygen hits hydrogen peroxide, the peroxide splits in half and forms two hydroxyl radicals.
Hydroxyl radicals are a hydrogen atom bonded to an oxygen atom with an unpaired electron this is what makes it a radical. Electrons always prefer being in pairs, so having an unpaired electron makes a compound very reactive. The hydroxyl radical can attach to the backbone of DNA deoxyribose , which can cause the DNA strand to break or a base pair to be released.
Both of these outcomes can be very harmful to the DNA or the cell. Our bodies, however, do not lie down and accept their fate - there are numerous defense mechanisms to protect against and mitigate the damage. Several enzymes travel around the DNA looking for this abnormality.
When they find such a bulge, they activate repair proteins that cut out the damaged part of the DNA and put in the correct base pairs. This whole process is called nucleotide excision repair. The effect of indirect DNA damage is harder to detect because transversion does not result in a distorted helix. The mechanism that repairs this kind of damage is called base excision repair.
Enzymes called DNA glycoslase remove a base pair misplaced by transversion; other enzymes then open up the DNA's backbone so that DNA building enzymes can come through and fill the gap with the correct base pair. Our bodies have mechanisms that help us in the long-term as well.
Direct DNA damage signals the production of additional melanin, so that the next time the skin is exposed to UV light, more can be harmlessly absorbed by the melanin. This means that anytime you become more tan after being outside, there was direct DNA damage! So give your DNA a break and apply sunscreen the next time you're enjoying the sun!
Note: I drew the pictures, so if you see any errors or would like another reaction displayed, please comment! Agnez-Lima, Lucymara F.
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