10. Suppose that the following enzymatic pathway is used to synthesize amino aci

Question

10. Suppose that the following enzymatic pathway is used to synthesize amino acid histidine in Neurospora

:

                    enzyme A                  enzyme B                  enzyme C

Precursor ------------> Imidazole ---------> Histidinol -----------> Histidine

These enzymes are encoded by three unlinked genes A, B and C respectively. A prototrophic strain with

the genotyp ABC is crossed to an auxotroph (triple mutant abc).

A. What proportion of spores do you

expect to produce a colony on a minimal medium without any of these three substances?

The answer is 1/8. Please show you work on how to acquire this quantity.

B. What proportion of spores do you expect to produce a colony on a minimal medium supplemented with histidinol? The answer is 1/2.. please show your work

Explanation / Answer

The given pathway is needed to utilize aminoacid histidine in the Neurospora. Three enzymes A, B, C are necessary to obtain the histidine end product. It is given that prototrophic strain with ?the genotyp ABC is crossed to an auxotroph (triple mutant abc). ? Then the progeny obtained will be the following ABC AbC ABc aBC abC abc Abc aBc For the spore to survive all the three enzymess should be functionalin nature. Therefore only the spore with all the three genotypes ABC will survive in the colony. Total progeny are 8, and the progeny with all the three genotypes ABC is one. So, the proportion of spores survive will be 1/8. B). When histidinol is supplemented to the medium, only the enzyme C is sufficient to produce histidine. Therefore the number progeny with enzyme C are 4. Total progeny are 8 proportion of spores expected to produce a colony on a minimal medium supplemented with histidinol = 4/8 = 1/2

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