There are very many photochemical reactions: Up to 50–100 mutagenic reactions on DNA per second might occur in a skin cell during exposure to sunlight, but are usually corrected within seconds by photolyase reactivation or nucleotide excision repair. (That's just DNA, excluding O2 and other cell constituents)
chemist ref and wiki ref
Molecules that absorb photons can adopt transient excited states whose chemical and physical properties differ greatly from the original compounds.
New chemical species arise, which can cleave, glue in different places, change structures, combine with other molecules. They can also transfer electrical charge, protons and hydrogen atoms, to other atoms.
The elevated states of compounds are stronger acids and reductants than the ground states.
Free radicals from UV have an indirect effect on DNA:
Superoxide is partitioned by the enzyme Superoxide dismutase into O2 and H2O2, eventually giving way to hydroxyl radicals OH.
Hydroxyl radicals can attack the deoxyribose backbone and bases, potentially causing a plethora of lesions that can be cytotoxic or mutagenic.
UV's can also detach iron atoms in the mitochondria, which can breach the mitochondria and cause havoc inside the cell.
Cyclobutane pyrimidine dimers are the most common DNA mutation cause by photons.
Two "T" bases forming thyamine dimer:
it can also be thiamine-cysteine.
UVB is a lot more reactive with DNA, and recent studies find that UVA travels deeper and can also cause CPD dimers.
Photochemistry in cells is a bit like throwing a spanner at a billiard table, reactions can occur with hundreds or thousands of organic compounds, one of those being DNA.
The following is the answer I posted prior to that:
Radiation/electromagnetic waves are an extended version of the visible light rainbow spectrum, which go from radio waves to X-rays and Gamma rays.
The lower energy waves (longer wavelengths) can raise electrons to higher energy levels in the atoms. The higher energy waves (shorter wavelengths) can knock electrons totally off of the atoms, leaving a positively charged atom (a positive ion) and an electron. Since electrons are often shared between two atoms of a molecule like an atomic glue, knocking an electron away can sometimes reconfigure or cleave molecules in two.
The ionisation line is also the Lyman-alpha line, which is the wavelength absorbed by hydrogen atoms, the smallest atoms. All radiation shorter than hydrogen absorption is known as ionising radiation.