Although DNA absorbs maximally at 260nm (UV-c does not survive passage through the atmosphere), significant absorption continues through the UV-b region to 315nm. Photons absorbed directly by DNA can modify covalent bonds within the macromolecule, forming two characteristic types of cross-link between adjacent pyrimidine (thymidine and cytosine) bases. The biological consequences of the formation of these cross-links between adjacent bases is interference with DNA replication and transcription, since DNA polymerase and reverse transcriptase enzymes are stalled or physically blocked by the structural changes in the DNA molecule (RNA can also be photodamaged by UV radiation, through the formation of cross-links between adjacent uracil molecules and the formation of cross-links with tRNA molecules);
Other chromophoric molecules (NADH, NADPH, flavins, proteins, unsaturated lipids, and other molecules with p-electron systems within the cell) particularly absorb in the UV-a region. Once the chromophores absorb energy they become "photosensitized", and thereby can either pass the energy directly to DNA or react with an intermediary molecule to generate reactive intermediates. Photosensitized reactions are thought to result in single- and double-strand DNA breaks, DNA-protein cross-links, and photohydrates (the addition of a water molecule to a cytosine or thymine molecule). The combined result of accumulated DNA damage in living cells from UV radiation, whether by direct or indirect (photosensitized) mechanism, is that DNA replication or transcription is disrupted, cell division may cease, and mutations in essential genes may cause cell death.
Less energetic and therfore "sublethal" or non-DNA injuries may not lead directly to cell death. However, they can significantly affect the growth and reproduction of cells, and they can act synergistically with the effects of DNA damage to increase lethality (i.e. affecting membrane-permeability and transport of molecules, including amino acids and sugars) into the cell; many of the electron transport chain components (located within the cell membrane) including menaquinone, riboflavin, and porphyrins, can absorb UV radiation. Damage to these molecules interrupts the electron transport chain and disrupts proton gradients across the cell membrane;

p.208-209; in ....
Moran M.A., Zepp R.G.; 2000; UV Radiation Effects on Microbes and Microbial Processes; in Kirchman D.L. (ed); Microbial Ecology of the Oceans; Willey-Liss Inc.; New York USA;