Concept explainers
To determine: Which type of nutrient medium would a double mutant with argG and argE grow on.
Introduction: Beadle and Tatum selected Neurospora crassa, the bread mold for their experiments. They allowed the growth of cultures in a nutrient-rich medium and then subcultured individual fungal cells by placing them on minimal medium. This process enables the identification of cells that lost the capability to synthesize compounds essential for growth. They aimed to study the capability to synthesize arginine amino acid.

Explanation of Solution
Beadle and Tatum selected mutants that can survive and grow on minimal medium with arginine. This resulted in a set of independent mutants that cannot synthesize arginine, and each of these mutants could be mapped genetically to different chromosomal positions. This research described four genes, namely argE, argF, argG, and argH. The specific lesion in each mutant can be determined by adding specific intermediates in the arginine biosynthesis pathway. Growth should be observed if the mutation influences the enzyme that occurs earlier in the biochemical pathway than the intermediate added to the minimal medium. No growth should be observed if the mutation influences an enzymatic step that occurs after the intermediate added.
The ArgE mutants are obstructed at the first step in the pathway. Therefore, the double mutant strain with argG and argE would grow only on nutrient medium supplemented with all the reaction intermediates.
To describe: What can a double mutant say about the order of the genes.

Explanation of Solution
Generally, these types of double mutants enable researchers to conclude which gene exists first in the pathway. The double mutant will appear like an organism with a mutation at the initial point in the biological pathway.
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Chapter 15 Solutions
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