Question

1.The His- phenotype of an Salmonella strain is caused by a single-nucleotide deletion. For this strain, we can expect to find His+ revertants after treatment with:

Group of answer choices

A.X-rays

B. Acridine orange (intercalating agent)

C. Nitrous acid (deaminating agent)

D. This mutation is very unlikely to be reverted

2) The His- phenotype of an Salmonella strain is caused by a transition mutation. For this strain, we can expect to find His+ revertants after treatment with:

Group of answer choices

A. X-rays

B. Acridine orange (intercalating agent)

C. Nitrous acid (deaminating agent)

D.This mutation is very unlikely to be reverted

3.The His- phenotype of an Salmonella strain is caused by a very large deletion. For this strain, we can expect to find His+ revertants after treatment with:

Group of answer choices

A.X-rays

B.Acridine orange (intercalating agent)

C.Nitrous acid (deaminating agent)

D.This mutation is very unlikely to be reverted

4.The Ames test is used to assess the ability of a chemical to cause mutations. The Ames test measures:

Group of answer choices

A.The rate of reversion of His- auxotrophic bacteria to wild type.

B.The rate of mutation of wild type bacteria to His- auxotrophy.

5.The Ames test was used to assess the mutagenic properties of a new chemical, Compound X. For this test, a Salmonella mutant that is auxotrophic for histidine was mixed with rat liver extract and grown on two separate plates of media lacking histidine. On the first plate, Compound X was also added, and a large number of colonies grew. On the second plate, no chemical was added and 2 colonies grew. These results suggest that Compound X is mutagenic.

True or False

Answer 1

You have asked 5 questions here. As per Chegg Q and A Guidelines, I am answering the first question only.

As mentioned in the question, the His- strain is developed by a single nucleotide deletion. Thus to convert a His+ to His- strain a single deletion mutation is required. Hence to convert a His- strain to a His+ strain, a nucleotide addition is required.

Now, let us analyze the mutagens that are provided in the options.

1. X-rays: X-rays are high-energy radiations. These radiations induce Thymine-Thymine dimers and thus inhibit the replication of the cell. The cell tries to repair it using Nucleotide Excision Repair mechanism. This method removes these thymine dimers thereby causing deletions and most often frame-shifts. Nucleotide Excision Repair doesn't add new nucleotides to the existing chain. Hence the desired revertants will not be obtained by this mutagen.

2. Acridine orange: Acridine orange is an inter-calating agent. It binds to DNA in a planar fashion. This leads to errors in replications and thus mutations. The mutations induced by an inter-calating agent can be additions as well as deletions. Hence acridine orange can be used to revert His- to His+.

3. Nitrous acid: Nitrous acid is a deaminating agent. It acts on cytosine converting it to uracil. Thus, the modified base binds to adenine instead of guanine. This leads to a substitution mutation. Similarly, it might also act on adenine by converting it to hypoxanthine. The hypoxanthine binds to cytosine instead of thymine again leading to a substitution mutation. Thus, nitrous acid induces substitutions in the DNA bases. It does not add new bases and hence addition mutations are not possible with this mutagen. Thus, His- strains will not revert to His+ strains upon treatment with nitrous acid.

Thus, an appropriate mutagen for reverting His- to His+ i.e. inducing additions would be a mutagen that is capable to inducing additions. Of the given options, Acridine orange meets this demand.