Question

TABLE 2: AMES TEST RESULTS FOR SALMONELLA Salmonella Strain A Data Plate Substance Toxic? Mutagenic? Zone Inhibition (mm) (table avgs.) (class avg.) 0 0 0 0 # Colonies/Plate (table avgs.) (class avg.) 72 77 i 64 | A1 Negative Control (DH20) OZ - - 16 i 5 5 167 i 160 i 165 164 NaNO2, 1 mg yes yes -- . - A3 10 | 11 | 11 NaNO2, 2 mg 140 150 140 yes -- -- A4 NaNO2, 25 1 15 18 173 175 180 -- 3 mg -- - AS Soap | 45 | 47 | 50 -- - - A6 Herbicide 32 · 58 22 43 | 48 36 63 75 71 000 -- - - - A7 Acridine Orange 20 37 11 70 - - -- - 0.5 ug -- - -- - - Salmonella Strain B Data Plate Substance Toxic? Mutagenic? Zone Inhibition (mm) (table avgs.) (class avg.) 0 0 0 # Colonies/Plate (table avgs.) (class avg.) 13 1 12 ! 15 13 B1 Negative Control (dH20) --- NaNO2, oroon 10 | 9 | 12 no -- 1 mg -- BS - yes - B6 Soap | 55 | 54 55 | Herbicide 17 | 10 | 10 Acridine Orange Oioio 0.5 ug | 11 | 12 4 yes --- B7 0 86 | 120 | 100 102T yes - -- -

1. The estimated spontaneous mutation rate for Salmonella strain A is: _____71____________________.

2. The estimated spontaneous mutation rate for Salmonella strain B is: _______13__________________.

3. The estimated spontaneous mutation rate for the E. coli strain is: ___________29______________.

4. Why might Salmonella strain A and B and the E. coli strain have different spontaneous mutation rates? Think about which strain may have a better DNA repair system and how membrane permeability can affect mutation rates.
5. Based on the number of colonies per plate, when do you consider a test compound to be mutagenic?
6. What does it mean if a substance results in fewer colonies than the negative control?
7. What is the purpose of adding trace amounts of histidine or tryptophan to the top agar?
8. Was the soap mutagenic and/or toxic in either strain? Explain your reasoning.
9. Was the herbicide mutagenic and/or toxic in either strain? Explain your reasoning.
10. Why does acridine orange appear mutagenic in Salmonella Stain B but not in Salmonella Strain A? Why does sodium nitrite appear mutagenic in Salmonella Strain A but not in Salmonella Strain B. Explain.
11. Attach a graph of sodium nitrite dose vrs colonies per plate. Does the zone of inhibition change with the of dose of nitrite? Is the change proportional to dose?
12. Does your graph indicate that there is a relationship between number of colonies per plate and sodium nitrite dose? Explain.
13. Would you conclude the substance you brought from home is mutagenic?
14. How can you be certain the substance from home was not contaminated with prototrophic bacteria?   If it was contaminated, did it confound your interpretation of mutagencity?
15. If you were asked to set up an Ames Test for your chemical company, would you use either Salmonella strain A and/or strain B? Briefly explain your reasoning.

Answer 1

Spontaneous mutation is directly associated with the speed of transcription. During transcription, the dsDNA is converted ssDNA for RNA synthesis. ssDNA is more susceptible to damage compared thereto of dsDNA. So ultimately, higher the speed of transcription, higher rate of spontaneous mutation. Hence organisms with different rate of transcription have varying rate of spontaneous mutation frequencies.

DNA repair systems identifies and corrects the damages within the DNA. Higher the efficiency of DNA repair systems, lower the speed of spontaneous mutation. therefore the organism with lower rate of spontaneous mutation features a better DNA repair system. So, the Salmonella Strain B features a better DNA repair systems.

.) Ames test (Salmonella typhimurium reverse mutation assay) is employed to spot whether a given chemical substance can cause DNA damage. The strains used for Ames test should have a suitable rate of spontaneous mutation. When the spontaneous mutation frequency falls outside a suitable range the genetic integrity of the organism is taken into account to be compromised, which might drastically affect the test results.

Hence, Strain B are often used for Ames test to screen the chemicals for his or her mutagenecity.....