If yeast colonies are exposed to UV-C light over generations, they will develop a resistance to the stress. When compared with the strain that was never exposed to UV-C the survival of the UV-C exposed yeast colonies will be greater because they will have evolved over generations of UV exposure to develop Ultraviolet resistance.

• Eukaryotic Similarity Human DNA and yeast DNA are incredibly similar, they have nucleus, they both divide their cells and they both have a shared ancestry

• The human DNA, when damaged, can repair itself using a process called nucleotide excision repair

• Yeast cells can repair their DNA damage using Photoreactivation

• Rapid Generation Time - Yeast can divide and grow their population in hours which allows us to study their evolution

• DNA damage in humans can cause cancer, premature aging and neurological disorders and UV rays can be one of the causes for DNA damage

• In terms of UV light, there are three different types. UV-A, UV-B and UV-C light.

• The UV light that enters the Earth;s atmosphere is UV-A and UV-B light

• UV-C rays come from the Sun but most of these rays are blocked by the ozone layer

• The main impacts of UV-C rays are photokeratitis, burning of the skin and skin cancer

• A project in the California state fair 2004, showed that the colonies put under the UV light were significantly smaller and the ones with a longer exposure time showed depressing results

• Ultraviolet resistance is a phenomenon that occurs when a substance like yeast begins to form a “resistance”

• The changes in the yeast colonies can be genotype or phenotype

• Mutations occur when the cell divides and changes its “code”

• Mutations can also occur from environmental stressors, like UV light, alcoholic stress, osmotic stress

• There are 8 types of mutations in total (substitution, insertion, deletion deletion, duplications, inversions and translocation

Manipulated - The yeast cells that will be exposed to UV rays  

Responding - Changes (number, morphology, etc.) in the yeast colonies that are stressed 

Controlled - Degree of dilution, time of exposure to UV-C light, temperature and duration of incubation, amount of yeast culture plated, type of agar gel plates (10-6 as the dilution, 40 seconds of UV exposure time, 30oC, 48 hours, 0.1ml, plates from Dalynn Biologicals).

Step 1: Serial Dilution and testing for best dilution

1. Take 100ml of warm distilled water.
2. Add the 1 grams of yeast to the water
3. Let this mixture sit for 20 min
4. Take 10ml of this yeast mixture and pour it into a test tube
5. Put 9ml of sterile water into 6 other tubes
6. Label each tube with numbers 1-6
7. Take 1ml of yeast mixture and pour it into the tube labelled one
8. Mix this mixture 
9. Take 1ml of the new mixture and add it into the tube labelled 2
10. Mix this mixture
11. Take one ml of this mixture and add it into the tube labelled 3
12. Repeat this process for all 6 tubes
13. Now take the tubes labelled 4, 5 and 6
14. Pour 0.1 ml of each mixture into 3 separate plates
15. Spread the mixtures on the plate
16. Label the plates according to the type of mixture that was put in (4, 5 or 6)
17. Incubate the plates for 48 - 72 hours
18. Check which plates shows the best growth for counting

Step 2: Testing for Survival Rate 

1. Take 0.1 ml of the 10-6 diluted mixture and plate it onto 6 different plates labelled - 0s, 20s, 30s, 40s, 50s and 60s
2. Then expose the 5 plates labelled - 20s, 30s, 40s, 50s and 60s to the UV-C light respectively.
3. The plate labelled 0s will not be exposed to the UV-C light.
4. Let the plates rest for 20 minutes
5. Flip the plates upside down and incubate them for 48 hours at 25-35oC
6. After 48 hours count the number of colonies on each plate and calculate the survival percentage against the plate that was not exposed to the UV-C light.
7. The time that gives a survival rate of 60-70% should be selected for the further experiment.

Step 3: Starter Culture and Generation G (G0)

1. Take 100ml of warm distilled water.
2. Add the 1 grams of yeast to the water
3. Let this mixture sit for 20 min
4. Perform serial dilution up to 10-6
5. Plate the diluted yeast mixture into 4 plates with 0.1ml of mixture in each plate (2 controlled, 2 manipulated (exposed to UV-C))
6. Take the 2 plates and expose under UV-C for 40 seconds.
7. Incubate the six plates for around 48 hours
8. After 48 hours count the colonies on each plate and record the data- this is “Generation  0” (G0) or ancestral culture.

Step 4 : Next Generations (G1-G5)

1. Take two jars and label them “Control” and “UV” and add 100 ml of sugar and nutrient solution to each  (recovery broth)
2. Grow the UV and Control lines parallely.
3. Harvesting: Once the colonies grow on that UV plate, they are the "Survivors."
   1. Use a sterile loop or a toothpick to scrape 10 of these colonies off the agar from both the UV plate and control plate..
   2. Drop them into a fresh recovery broth in jars labelled UV and Control respectively
4. Generation 1 (G1): Let them grow in the broth overnight until the liquid is cloudy (this allows the survivors to multiply).
5. Testing: Now, perform the same serial dilution and plating as before for both the lines. Expose the  G1 yeast UV plate for 40 secs as before.
6. Incubate both the plates for 48 hours.
7. Yeast grown after 48 hours is Generation 2
8. Repeat this process until generation 5

Step 5 : Testing for Resistance/Evolution

1. Plate two of Each the Control line and UV line while going from Generation 4 to Generation 5.
2. Expose both the Control and UV line cultures of Generation 4 to UV-C light for 40 seconds. 
3. Also, plate both the UV and Control culture without any exposure
4. Record the survival data of the yeast on all the plates obtained in Generation 5.

• The survival rate of the UV exposed yeast colonies increased over generations however not significantly, besides a small dip in Generation three before increasing
• In generation 5 (G5) when the control was exposed to UV the survival rate of the control colonies exposed to UV (86.00%) compared to the survival rate in the UV colonies (66.67%) was greater
• The survival rate of G5 the UV exposed line (66.67%) to that of G0 (37.75%) is considerably higher.
• Some of the plates had phenotypic changes that could indicate a mutation
• Size of the colonies were smaller in the UV line compared to the control line
• The UV exposed colonies showed slower growth when compared to the Control colonies

• While the UV exposed yeast did increase its survival rate, the increase compared to that of the control group was not considerable. 
• The survival rate of the control yeast (Gen 5) after being put under UV, compared to the Gen 0 survival rate shows a great increase
  • Yeast has become more resistant and evolved to the environmental factors that it was being subjected to (genotypes)
• When we compare the survival rate of Gen 0 to survival rate of G5 there is a considerable increase in survival percentage
  • This shows that the resistance is being built in the yeast
• In even a single packet, there are many yeast cells that are different. This means that when the yeast colonies are put under UV, only those that have the most vitamins will survive
• This can be linked to Charles Darwin’s theory, “survival of the fitness”
• In conclusion my hypothesis was correct and the yeast evolved to develop some resistance

The real-life applications of this project are that this project can be used for human DNA. Since the ozone layer is depleting, UV-C light is entering the Earth and we are being exposed to it. Yeast DNA and human DNA are incredibly similar so this study can be extended to human DNA to see if humans can become resistant to UV light. Furthermore, more research can be done using other stressors or even using other organisms such as bacteria.

A source of error in the project is that all living organisms are unpredictable, including yeast. This means that yeast can at times grow very little and at other times show surprising results this means that the results of this experiment could vary. Another error is that I tried my best to keep the temperature constant though it could have changed.

Effect of ultraviolet UV rays on yeast colony growth | science project. Science Buddies. (n.d.). https://www.sciencebuddies.org/science-fair-projects/project-ideas/BioChem_p036/biotechnology-techniques/UV-dna-damage 

https://csef.usc.edu/History/2004/Projects/S1316.pdf 

https://pmc.ncbi.nlm.nih.gov/articles/PMC7582305/

https://www.canada.ca/en/health-canada/services/sun-safety/what-is-ultraviolet-radiation.html 

https://study.com/learn/lesson/pyrimidine-bases-structures.html 

https://stemcellsjournals.onlinelibrary.wiley.com/doi/full/10.1634/stemcells.19-4-348

https://pmc.ncbi.nlm.nih.gov/articles/PMC2262034/

https://www.genome.gov/about-genomics/fact-sheets/Deoxyribonucleic-Acid-Fact-Sheet 

Mutations (Updated) 

https://www.corrosionpedia.com/definition/2424/ultraviolet-resistance-uv-resistance 

https://pmc.ncbi.nlm.nih.gov/articles/PMC9194483/ 

https://www.phys.ksu.edu/gene/d4.html 

file:///Users/sahilkiran/Downloads/mayui,+cpS.cerevisiaeT%20(1).pdf 

https://www.science.org/content/article/yeast-can-live-human-genes    Drinjakovic, J. (2016, April 6). Yeast against the machine: How yeast could improve diagnosis. Temerty Faculty of Medicine. https://temertymedicine.utoronto.ca/news/yeast-against-machine-how-yeast-could-improve-diagnosis#:~:text=Currently%20the%20only%20way%20to,to%20keep%20the%20yeast%20afloat 

webteam), www-core (Sanger. “Model Organisms: Yeast - What Have We Learnt from Yeast?” YourGenome, Unknown, www.yourgenome.org/theme/model-organisms-yeast/.  Accessed 2 Mar. 2026. 

Pizzul P, Casari E, Gnugnoli M, Rinaldi C, Corallo F and Longhese MP (2022) The DNA damage checkpoint: A tale from budding yeast. Front. Genet. 13:995163. doi: 10.3389/fgene.2022.995163 

I would like to acknowledge the several people that helped bring this project to completion. First and foremost, I would like to thank Dr. Garcia, my mentor who answered my questions and guided me throughout the project. Then, I would like to acknowledge my mom, for helping me buy the materials and giving me advice on my project. Last but not least, I would like to thank Aayan, my mentor who was there on a weekly basis to guide me through the project.