Why does UV radiation from the Sun cause skin cancer?
Rather than 'breaks' caused by high energy radiation, UV radiation causes chemical modifications of the bases ('letters') that make up DNA. There are four bases in the DNA alphabet abbreviated to A,T,G or C belonging to two classes. A &amp; G are guanines while T &amp; C are pyrimidines.
When you have pyrimidine bases next to each other on a DNA strand, they can react to form photoproducts or pyrimidine dimers. These block transcription and replication and need to be repaired by nucleotide excision enzymes (only route in mammals) or photolyase enzymes (plants, many other organisms). TT dimers are the most frequent but dimers with a C cause more trouble as they can be imperfectly repaired causing a transition mutation. If they don't get repaired in time they can cause stalled replication which causes all kinds of trouble and can result in mutations that lead to cancer. You can find more on the wiki page.