Ch 17 Effects Of Ultraviolet Light On Bacteria - Micro Lab

Front 1

solar ultraviolet radiation (UV)

Back 1

• part of the non-ionizing radiation spectrum
• divided into 3 types according to wavelength (A, B, C)
• the total range is 200-400nm
• can be bacteriocidal

Front 2

UV-A

Back 2

• emitted at wavelengths of 320-400 nm
• make up 95% of solar UV radiation
• indirectly causes DNA damage by activation of photosensitizer proteins

Front 3

UV-B

Back 3

• emitted at wavelengths of 290-320 nm
• directly absorbed by nucleic acids (DNA) to form pyrimidine dimers
• usually thymine dimers on the same DNA strand between 2 adjacent T molecules
• the dimer is covalently bound
• causes a bend or distortion of the DNA forcing DNA Pol to dissociate preventing replication or transcription

Front 4

UV-C

Back 4

• emitted at wavelengths of 200-280 nm
• the most damaging but absorbed by atmospheric ozone and oxygen before it can reach earth's surface
• absorbed by all cellular components

Front 5

What are the two methods of repairing DNA dimers discussed in this lab?

Back 5

1. light repair - photoreactivation (not in humans)
2. dark repair - nucleotide excision (in humans)

Front 6

light repair or photoreactivation

Back 6

• present in many organisms but NOT humans
• photolyase enzymes bind to pyrimidine dimers and break the covalent bond to restore the original pyrimidines
• light energy is used to break the covalent bond

Front 7

dark repair or nucleotide excision

Back 7

• present in humans
• light is not required
• endonucleases (RecA) excise the damaged DNA
• the gap is then repaired by DNA Pol and DNA ligase

Front 8

UV-crosslinker

Back 8

used as the UV light source to expose samples to a controlled amount of UV radiation at 254 nm

applications in the lab are:

• crosslinking of DNA/RNA to transfer membranes
• northern and southern blots
• photo-nicking of DNA
• RecA mutation screening
• sterilization of surface contaminants

Front 9

Which bacteria are used in this experiment?

Back 9

• E. coli (Gram -)
• S. aureus (Gram +)
• XL1 blue II (Gram - w/ Rec A mutation)

Front 10

What is XL1 blue II?

Back 10

A mutant strain of E. coli that does not have the Rec A enzyme. It cannot perform dark repair of thymine dimers.

It should not grow if exposed to UV light