Exercise 10-5 Micro Lab: Ultraviolet Radiation Damage and Repair

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Why is prolonged exposure to ultraviolet light damaging to cells?

because when DNA absorbs UV radiation at 254nm, the energy is used to form new covalent bonds between adjacent pyramidine dimers which distort the DNA molecule and interfere with DNA replication and transcription


Pyramidine dimers (3)

Which one is the most common?

1. cytosine-cytosine
2. cytosine-thymine
3. thymine-thymine (most common)


How does E. coli bacteria perform UV light damage repair? (2)

1. It performs photoreactivation (light repair)
2. Excision repair (dark repair)


photoreactivation (also known as light repair)

the repair enzyme, DNA photolyase, is activated by visible light (340-400nm) and simply monomoerizes the dimer by reversing the original reaction


excision repair (involves 4 enzymes)

1. The thymine dimer distorts the sugar-phosphate backbone of the strand.

2. This is detected by an ENDONUCLEASE (UvrABC) that breaks two covalent bonds -- eight nucleotides in the 5' direction from the dimer, and the other four nucleotides in the 3' direction.

3. A HELICASE (UvrD) removes the 13-nucleotide fragment (including the dimer), leaving single-stranded DNA.

4. DNA POLYMERASE I synthesizes a new strand by inserting the appropriate complementary nucleotides in a 5' to 3' direction to make the molecule double-stranded again.

5. Finally, DNA LIGASE closes the gap between the last nucleotide of the new segment and the first nucleotide of the old DNA and the repair is complete.


Why is UV light limited?

because it penetrates glass and plastic poorly


3 bacteria used in lab and their correlation to UV light

1. Bacillus - spore formers

2. Serratia marcescens - have genes involved in pigment formation which are susceptible to UV radiation

3. Chromobacterium violaceum - has the ability to use photoreactivation