Is DNA or protein the genetic material?

 Is DNA or protein the genetic material?

Objectives:

Students will learn:

about a study that compares the frequency of mutations arising from exposure to different wavelengths of light, to the absorption spectra of DNA and protein.

In the early 1930s, it was discovered that ultra-violet light (UV) could induce mutations in Drosophila. A few years later, a bacteriologist interested in the mutagenic effects of UV light decided to test which wavelength in the UV spectrum induced the most mutations. She performed her experiments on an auxotrophic (cannot synthesize a necessary organic compound) strain of the enteric (intestinal) bacterium Escherichia coli, that requires the amino acid histidine for growth.

Wild type E. coli can grow on minimal medium that includes a few salts and a carbon source. The bacteriologist knew that the E. coli his mutant used in her experiment could mutate back to prototrophy (have the same nutritional needs as the wild-type parent strain) and grow on minimal medium. She exposed cultures of E. coli to UV light of different wavelengths ranging from 220 nm to 320 nm. She then spread 109 cells from each culture on 5 petri dishes with agar that contained minimal medium. E. coli that mutated back to being prototrophic formed colonies on minimal medium, whereas the parental his mutant could not grow.

Table A. UV Light Mutations

Table A contains the results achieved by the bacteriologist for the number of mutants obtained after exposure to different wavelengths of UV light. These data are also in an Exel file that you can download.

The bacteriologist then decided to compare her results for the frequency of mutations to the absorption spectra of DNA and protein. In particular, she was interested in demonstrating that the genetic material was made of proteins.

How can this information indicate what composes the genetic material? Mutation is a change in the genetic material caused by a copying error, exposure to a mutagenic chemical, or in this case, chemical changes caused by exposure to radiation (UV light).

In order for the genetic material to be altered by UV light, it has to absorb it. Different substances absorb different wavelengths of electromagnetic radiation. For example, radiation in the visible light spectrum passes right through glass without interacting with it, while visible light absorbed by a black lab bench will cause it to heat up.

Therefore, by finding a correlation between known UV absorbance and the rate at which different frequencies of UV light cause mutation, it was hoped that this information could show which substance – DNA or protein – was being altered to produce mutation.

Activity

1. Download the associated Exel file, where you will calculate data and draw graphs (see instructions below).

2. Calculate the average number of mutants generated by exposure for each frequency of UV light and enter the values in the Exel file.

3. Using Exel, plot three graphs that all have the wavelength of UV light on the x-axis, and for which the y-axis is each of the three measured variables in the table. Make sure that each graph has a title and that the axes are labeled.

4. Paste these graphs into a 1-page Word document that has your name at the top. Submit the Word file using the TurnItIn link that will be provided.

5. Discuss what the results mean in the context of what the genetic material might be.

REVIEW QUESTIONS

1. What was the researcher’s initial hypothesis concerning the nature of the genetic material?

2. Why did the researcher perform multiple replicates of the experiment?

3. Did the data support or refute the hypothesis?

4. Based upon these data, what would you conclude is the genetic material?