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Name: Sarah
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Grade:  9-12
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Date: October 2008

How does UV Radiation physically affect bacteria?

UV causes adjacent thymine molecules to react with each other to form TT dimers. These dimers prevent DNA replication. DNA repair enzyme can excise them but mistakes are made during this repair which produces mutations.

Ron Baker, Ph.D.

UV radiation generally carries out its biological effect by damaging nucleic acids. The chromophores in sugars and proteins do not generally absorb in wavelengths of UV that reach the Earth's surface, so those biomolecules are not UV-sensitive. The purine and pyrimidine bases in nucleic acids, on the other hand, do absorb UV. The added energy excites them to a higher electronic state. In particular, thymidine, the nucleobase in thymine, can dimerize when exposed to UV. This causes defects in transcription and replication.

Richard Barrans

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