Genetics Lab: Ultraviolet Lethality and Mutations in Yeast: Counting Colonies an

Question

Genetics Lab: Ultraviolet Lethality and Mutations in Yeast: Counting Colonies and Analyzing Results

Answer following questions:

1. With increasing doses of UV exposure, what trend would you expect to see on the yeast plates with regard to survival rates?

2. In terms of proximity to the UV source, how could we alter the procedure of this lab, in order to get more lethality and more mutations on the plates, without changing the length of exposure or the dilutions?

3. UV radiation causes the formation of dimers (covalent bonding between adjacent nucleotides) involving which bases?

4. Describe 2 repair systems which can repair the type of DNA damage described in #3 above, including how each system works, including the names of any enzymes and how they act to repair the damage.

5. We plated an ade- strain and exposed the cells to UV. What is the genotype and phenotype of the resulting yeast colonies which we counted as

Explanation / Answer

With increasing doses of UV exposure, we can expect to see gradual reduction in survival rates on the yeast plates. To alter the proximity to the UV source in order to get more lethality and more mutations on the plates you can vary or increase in the intensity of the UV source and you can alter the distance of exposure of the plates to the UV source. Majorly UV radiation causes (pyrimidine dimers) the formation of thymine dimers occasionally it can also cause cytosine dimers. Mechanisms to repair UV induced DNA damage by formation of thymine dimers are as follows: Light repair or photoactivation repair- Enzyme photolyase is activated by blue light of visible spectrum breaks the dimer, restoring the normal base pairing. Dark repair or Excision repair

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