NASA SOHO Mission,Yohkoh
science team
Ecological impacts of
UV-B radiation
Organismal effects and responses
Molecular photobiology
Biologically significant molecules found in all living things absorb UV-B, leading to damage that may or not be repairable. Nucleotide bases within nucleic acids, such as adenine, guanine, thymine, and cytosine in DNA, absorb UV-B as do proteins, porphyrins (such as hemes and chlorophyll), carotenoids (the red, yellow and orange pigments in vegetables and fruits), steroids, and quinones. Photoproducts, such as pyrimidine dimers, are formed as a result of UV-B absorption and they block DNA replication. Therefore, UV-B damaged cells (and viruses) can lose their ability to reproduce. Indirect damage to DNA in response to UV-B exposure is also possible because non-living molecules that absorb photons can form free-radicals and active oxygen species, which can in turn react with DNA.DNA repair mechanisms can allow a UV-B damaged cell to become functional again and include photoreactivation, excision repair, postreplication repair, and SOS repair. Photoreactivation and excision repair are enzyme mediated repair mechanisms with the photoreactivation repair requiring light (between 330 and 600 nm) and excision repair occurring in the dark. SOS repair is thought to be the repair mechanism that causes ultraviolet-induced mutagenesis.
