Objectives: Mutagenic Effects of Ultraviolet Light and Photoreactivation Exercises 2-13 and 10-5

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Understand how UV light harms bacteria. (Understand the mode of action of ultraviolet light on bacterial growth.)

UV light causes 2 pyrimidines (cytosine or thymine) that are next to each other on the bacteria's DNA strand to dimerize. If all the pyrimidines pairs on the DNA strand dimerized, the DNA can't replicate

Mutigen breaks A-T bonds results in T's bonding together


Differentiate UV-A, UV-B and UV-C and their effects on living cells.

longest wavelength UVA => UVB => UVC shortest wavelength: the shorter the wavelength the more damage done to living cells

UV-A- 315 to 400 nm longest wavelength
UV-B - 280 to 315 nm
UV-C -100 to 280 nm most detrimental to bacteria


Understand the effects of UV irradiation on microorganisms that allow it to be used as for sterilization.

It is often lethal. Sometimes UV sterilization may not kill the bacteria outright but the bacteria will not be able to reproduce, spread, or cause disease.


Understand why some bacteria are less susceptible to UV radiation than others. Or in other words understand why UV light is less successful at killing Bacillus-spp. than Serratia marcescens

The presence of endospores makes Bacillus spp. more resistant. Thick wall doesn't allow UV penetration.


Understand why UV irradiation has limited application for sterilizing materials.

It only works on flat surfaces. If the surface is not flat you have disinfection not sterilization. In addition, UV radiation doesn't go through glass or plastic.

It is close to visible light and is reflected by common materials. Has trouble going through glass and only be used on a flat surface because microorganisms can survive in microshadows.


spontaneous mutations

have an unknown cause "seem random"


Define and describe photoreactivation.

bacteria exposed to visible light which accelerates repair


Define and describe excision repair (dark repair) of DNA.

Repair of damaged DNA by chemical cut and paste enzymes that cut mutations and paste appropriate groups to repair


Interpret results of our experiment: How does UV irradiation affect Chromobacterium violaceum, Serratia marcescens and Bacillus subtilis var. niger? How does the ability to complete photoreactivation change those effects?

Old Bacillus subtilis var. niger was able to form spores ; Serratia marcescens groups with non red colonies show that photoreactivity could work better on some mutations then others. Chromobacterium was more susceptible to UV light.


Understand why a section of each experimental plate was not exposed to UV irradiation. (Understand why a portion of the plate remained covered with the cardboard.)

They were our controls


induced mutations

have a known cause like higher radiation