UV Light

Thymine dimers

Block base pairing between thymines in one strand of DNA & adenines in another strand of DNA or RNA causing cell death

Endospore formers

Bacillus and Clostridium

Controls/no UV exposure results

Lawn of bacteria

UV exposure, no barrier results

Colonies or no growth; longer UV times have less growth indicating more death; cultures containing endospores die slower

Plate lid or bibulous paper results

Lawn slightly less dense than control; growth slightly less than control (lawn) and much greater than UV no barrier (colonies) indicating UV killed small amounts of cells, so UV poorly penetrates lid/paper

Vegetative bacteria UV results

UV quickly kill vegetative bacteria as it easily penetrates cell envelopes to damage DNA

Endospore former UV results

UV slowly kills endospores as it poorly penetrates thick dense outer coat to damage DNA

Processes UV blocks

Transcription and translation