You join a food chemistry lab where the researchers have found two new additives
ID: 217842 • Letter: Y
Question
You join a food chemistry lab where the researchers have found two new additives that are quite good at preventing food from spoiling. Before putting these chemicals out on the market, they decide that they should probably make sure there are no harmful effects to humans. They do an Ames test and observe the following results. Needless to say, they decide not to use the additives on food.
# of colony forming units on plates lacking histidine
Bacterial Strain
his- mutation
Untreated
Treated with Additive A
Treated with Additive A and liver extract
Treated with Additive B
Treated with Additive B and liver extract
TAF
Frameshift
1.6 ± 0.5
1.7 ± 0.1
3.0 ± 1.0
2.2 ± 0.4
1239 ± 28
TAT
T:A changed to A:T
9.3 ± 3.0
10 ± 4.0
12 ± 3.2
11 ± 2.8
12 ± 2.2
TAG
A:T changed to C:G
0.7 ± 0.3
621 ± 24
654 ± 31
1.0 ± 0.5
1.2 ± 0.6
A. What kind of mutation is caused by additive A? Name one specific type of mutagen that A could be. Explain your reasoning.
B. What kind of mutation is caused by additive B? Name one specific type of mutagen that B could be. Explain your reasoning.
C. Explain why the numbers of colonies on the plates of the TAF bacteria treated with additive B are different with and without liver extract whereas the numbers of colonies on the plates of the TAG bacteria treated with additive A with and without liver extract are not different.
D. As a positive control that you can in fact mutate bacteria, you treat wild-type (his+) bacteria with nitrous acid to induce hypoxanthine and uracil mutations in the genome. What type of chemical change is this?
E. To your surprise, the wild-type (his+) bacteria treated with nitrous acid do not contain the high number of hypoxanthine or uracil residues that you expected, given the number of adenines and cytosines in this bacterium’s genome. What specific process might be working to reduce the frequency of these mutations?
# of colony forming units on plates lacking histidine
Bacterial Strain
his- mutation
Untreated
Treated with Additive A
Treated with Additive A and liver extract
Treated with Additive B
Treated with Additive B and liver extract
TAF
Frameshift
1.6 ± 0.5
1.7 ± 0.1
3.0 ± 1.0
2.2 ± 0.4
1239 ± 28
TAT
T:A changed to A:T
9.3 ± 3.0
10 ± 4.0
12 ± 3.2
11 ± 2.8
12 ± 2.2
TAG
A:T changed to C:G
0.7 ± 0.3
621 ± 24
654 ± 31
1.0 ± 0.5
1.2 ± 0.6
Explanation / Answer
Remember that Ames test uses bacteria strains with different type of mutation in the histidine gen. Those that have a mutation after the additive was added and return the gen to its original sequence can grow colonies. With this in mind lets try to answer each item.
A) Transversion. This is the type of mutation occurred, when purins are replaced by pyrimidines.
We can infer this looking at the colonies that grew with additive A in the TAG strain with or without liver extract, compared to the untreated plate.
It could be caused by a chemical changing the specifity of hydrogen bonding, which produces deamination of adenine and results in formation of hypoxanthine, the pairing behaviour of which is like guanine. Hence, it pairs with cytosine instead of thymine replacing AT pairing by GC pairing. (For example Nitrous Oxide(HN02).
B) Additive B is clearly causing a Frameshift mutation, we can see it in the colonies that reversed in the TAF strain (1239!) compared to 3 untreated.
This type of mutation could be caused by intercalating agents, molecules which can be intercalated between the base pairs of DNA; distort the DNA and results deletion or insertion after replication of DNA molecule. Due to deletion or insertion of intercalating agents, there occur frameshift mutations (For example Acridin Orange)
C)These differences can be explained because additive B may not be mutagenic itself, but their metabolic products can be. This is why rat liver enzymes are added, in an attempt to mimic the metabolic processes in humans.
In TAG case, both essays are saying the same: Additive A is mutagenic and their metabolic products may be as well.
D)This is the first type of mutation seen in answer A). Its a chemical changing the specifity of hydrogen bonding. Causes transversions.
E)To reduce the frequency of these mutations, repair mechanisms might be working in order to maintain the DNA integrity.
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