Question

ara+ and lac+ are genes required for metabolizing the sugars arabinose and lactose, respectively, and ala+ and leu+ are genes required for synthesizing the amino acids alanine and leucine. You are given the following strains:

Strain 1: leu- ala+ ara+ lac+
Strain 2: leu+ ala- ara+ lac-
Strain 3: leu+ ala+ ara+ lac-
Strain 4: leu+ ala- ara- lac-

Which of these strains will grow on minimal media with glucose?

	A.	2, 4
	B.	2, 3, 4
	C.	3
	D.	1, 3
	E.	1

ra+ and lac+ are genes required for metabolizing the sugars arabinose and lactose, respectively, and ala+ and leu+ are genes required for synthesizing the amino acids alanine and leucine. You are given the following strains:

Strain 1: leu- ala+ ara+ lac+
Strain 2: leu+ ala- ara+ lac-
Strain 3: leu+ ala+ ara+ lac-
Strain 4: leu+ ala- ara- lac-

Which of these strains will grow on minimal media with arabinose?

	A.	1
	B.	2, 3, 4
	C.	2, 4
	D.	3
	E.	1, 3

ara+ and lac+ are genes required for metabolizing the sugars arabinose and lactose, respectively, and ala+ and leu+ are genes required for synthesizing the amino acids alanine and leucine. You are given the following strains:

Strain 1: leu- ala+ ara+ lac+
Strain 2: leu+ ala- ara+ lac-
Strain 3: leu+ ala+ ara+ lac-
Strain 4: leu+ ala- ara- lac-

Which of these strains will grow on minimal media with lactose + leucine?

	A.	3
	B.	1, 3
	C.	1
	D.	2, 3, 4
	E.	2, 4

Answer 1

1. Which of these strains will grow on minimal media with glucose?

Strain 1: leu- ala+ ara+ lac+
Strain 2: leu+ ala- ara+ lac-
Strain 3: leu+ ala+ ara+ lac-
Strain 4: leu+ ala- ara- lac-

Ans. For any strain to grow in minimal media it must possess all wild type genes i.e. genes for amino acid synthesis, genes for metabolizing carbon source, etc. Therefore, the strains mutant for gene of any amino acid will not grow; in this case the strains with either leu- or ala- will fail to grow. Only the strains with leu+ and ala+ will grow. Since the question is asking about growth in media with glucose but provides information about genes for arabinose and lactose catabolism we have to assume that there could be two types of conditions.

Condition I - Genes for glucose catabolism is of wild type.

in this case Strain 3 will grow. Presence or absence of arabinose pr lactose operon genes will not affect growth. In that case correct option will be C. 3

Condition II - Genes for glucose catabolism is of mutant type i.e. absent.

in this case none of the given strains will grow.

2. which strain will grow in minimal media with arabinose?

Strain 1: leu- ala+ ara+ lac+
Strain 2: leu+ ala- ara+ lac-
Strain 3: leu+ ala+ ara+ lac-
Strain 4: leu+ ala- ara- lac-

Ans. D 3. For any strain to grow in minimal media it must possess all wild type genes i.e. genes for amino acid synthesis, genes for metabolizing carbon source, etc. Therefore, the strains mutant for gene of any amino acid will not grow; in this case the strains with either leu- or ala- will fail to grow. Only the strains with leu+ and ala+ will grow. Moreover, the strain must have genes to metabolize arabinose (ara+) as this was the only carbon source supplied in the media. So it is only Strain 3 which possess all the necessary genes and hence only strain 3 will grow in minimal media with arabinose. Presence or absence of lactose metabolizing genes doesn't affect the growth in this case.

3. Which of these strains will grow on minimal media with lactose + leucine?

Strain 1: leu- ala+ ara+ lac+
Strain 2: leu+ ala- ara+ lac-
Strain 3: leu+ ala+ ara+ lac-
Strain 4: leu+ ala- ara- lac-

Ans. C 1. For a strain to grow in media with lactose and leucine it should have genes to metabolize lactose. The genes responsible for synthesizing leucine can be or can not be present as leucine is being supplied through media, however genes for other amino acids should be present. Therefore, it is Strain 1 only which possess genes for lactose catabolism and will survive in the media. Here, absence of genes for leucine synthesis doesn't affect growth as it is already supplied. Presence or absence of arabinose metabolizing genes doesn't affect the growth in this case.