UV Labs: Summary of Major Concepts
Do some research and answer the last three questions for homework
(one page or less and please remember to cite your sources).
Due_____________________
I. Serratia Lab
A. Ultraviolet (UV) radiation damages DNA.
1. The most common form of UV-induced damage are thymine dimers (which are created by
UV-induced covalent bonds between adjacent thymine bases in a strand of DNA).
2. Such damaged DNA cannot be transcribed or replicated.
3. Thus, cells with damaged DNA will die and will not reproduce.
4. Prediction: longer exposures to UV light will lead to fewer bacterial colonies on the plates.
B. Clearly, it is not good to have damaged DNA. Thus, cells have many DNA repair enzymes to fix
damaged DNA.
1. Most repair is error-free.
a. In other words, the damaged DNA is fixed perfectly and has no changes compared to non
UV-irradiated cells (e.g., it is the same as normal DNA).
b. Prediction: If error-free repair occurs following UV treatment, the cells will divide
normally and produce normal numbers of red colonies.
2. A small amount of repair is error-prone. This leads to mutations in the DNA.
a. In error-prone repair, the damaged DNA is repaired, such that the cell survives and can
reproduce. However, to repair the DNA, the damaged section is cut out and replaced with
a random string of DNA nucleotides. Thus, the DNA sequence of the repaired section is
changed. In other words, the process of repairing the DNA has made a mutation!!
b. Prediction: Because dozens of gene products are necessary to make the normal red
pigment, there is a good chance that the mutation will occur in one of the pigment genes.
Thus, a cell with such a mutation will divide but give rise to a white colony (absence of
pigment means white in color).
The Spectrum of Electromagnetic Radiation
C. How does DNA repair in bacteria incubated in the dark after UV treatment compare to those
incubated in the light after UV treatment?
1. Observation: there is more growth (e.g., more colonies) and fewer mutations in the lightincubated cultures. In other words, the dark-incubated cultures have fewer overall colonies
and proportionally more white colonies than the light-incubated bacterial cultures.
2. Conclusion: DNA repair is much more effective (more apt occur and to be error-free) in the
light than in the dark following UV treatment. This is called photo repair. In fact, one of the
enzymes involved in photo repair uses the energy of visible light to break apart the thymine
dimers in the damaged DNA, thus returning the DNA to its normal condition!
II. UV Bead Lab
A. Your exposure to UV radiation (by being in the sun or visiting a tanning salon) can also damage
the DNA in your skin and eyes.
1. Repeated exposure of your eyes to UV radiation can lead to cataracts and other eye disorders.
2. We all know that too much sun can give us a sunburn, which is an indicator of skin damage
caused by the UV radiation emitted by the sun. More dangerous, though, is that the UV
radiation may have damaged the DNA in your skin cells.
3. As described above, DNA damaged by UV radiation could result in a mutation. Thus, you
will have just increased your chance of developing skin cancer.
4. Malignant melanoma of the skin is one of the most aggressive of all cancers and something to
be avoided if possible!!
B. However, there are things we can do to protect ourselves from UV radiation, which you tested
using the UV beads. UV sunglasses or goggles can protect our eyes, and sunscreen, opaque
fabrics, tinted glass, etc., can protect our skin.
C. The baseline level of pigmentation in different groups of people is genetically-determined. People
with dark skin have more of the protein melanin in their skin then do lighter-skinned people.
These baseline levels of pigmentation arose though evolution, dependent upon the daily exposure
levels of our ancestors to UV radiation.
1. Chimpanzees are believed to be the closest living relative of humans. It is interesting to note
that chimps have light-colored skin under all their dark hair!
2. Nina Jablonski and others have proposed that our earliest ancestors, who lived in equatorial
regions, developed darkly pigmented skin as they lost their thick coat of dark, protective hair.
3. Subsequently, when different groups moved away from the equatorial regions, their
pigmentation levels decreased over time.
D. All organisms on Earth are protected from lethal doses of UV radiation by the ozone in the
atmosphere
1. What effects do you think the thinning of the ozone layer has had or will have in the future on
life on Earth?
2. What actions/events do you think are responsible for the recently-observed thinning of the
ozone layer?
3. What are you willing to do about the thinning of the ozone?