Ultraviolet radiation (UV radiation) cross-links adjacent pyrimidines on the same DNA strand, forming pyrimidine dimers, usually thymine dimers. Figure 3 shows the structure of a thymine dimer and illustrates how it interrupts base-pairing between the two DNA strands. These dimers block DNA replication because the replication machinery cannot tell which bases to insert opposite the dimer. As we will see, replication sometimes proceeds anyway, and bases are inserted at random. If these are the wrong bases, a mutation results.
Ultraviolet radiation has great biological significance; it is present in sunlight, so most forms of life are exposed to it to some extent. The mutagenicity of UV radiation explains why sunlight can cause skin cancer: Its UV component damages the DNA in skin cells, which leads to mutations that sometimes cause those cells to lose control over their division.
Given the dangers of UV radiation, we are fortunate to have a shield-the ozone layer-in the earth's upper atmosphere to absorb the bulk of such radiation. However, scientists have noticed alarming holes in this protective shield-the most prominent one located over Antarctica. The causes of this ozone depletion are still somewhat controversial, but they probably include compounds traditionally used in air conditioners and in plastics. Unless we can arrest the destruction of the ozone layer, we are destined to suffer more of the effects of UV radiation, including skin cancer.
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