Suppose your future self is working in a human genetics laboratory that studies
ID: 61004 • Letter: S
Question
Suppose your future self is working in a human genetics laboratory that studies causes and treatments for eye cataracts in newborns. This disease is thought to be caused by a deficiency in the enzyme galactokinase, but the human gene that encodes this enzyme has not yet been identified. You remember your cell bio class from college and recall that there are strains of baker's yeast which contain a mutation called gal1– in its galactokinase gene (not to get too meta). Because this gene is needed to metabolize galactose, the gal1 mutant yeast strain cannot grow in galactose medium. Knowing that all living things evolved from a common ancestor and that distantly related organisms often have homologous genes that perform similar functions, you wonder whether the human galactokinase gene can function in yeast. Because you rock, you decide to pursue this idea with of experimentation. You isolate mRNA gene transcripts from human cells, use reverse transcriptase to make complementary DNA (cDNA) copies of the mRNA molecules, and ligate the cDNAs into circular plasmid DNA molecules that can be stably propagated in yeast cells (although this is bacteria, the idea is the same). You then transform the pool of plasmids into gal1– yeast cells so that each cell receives a single plasmid. What will happen when you spread the plasmid-containing cells on Petri dishes that contain galactose as a carbon source? How can this approach help you find the human gene encoding galactokinase?
Explanation / Answer
According to the given data, the human gene that encodes the enzyme galactokinase is unknown. Galactokinase is the enzyme that allows the yeast to survive on galactose medium. Thus, the yeast that lacks this enzyme cannot grow on galactose medium.
In the given case, the human gene that encodes the galactokinase enzyme is inserted into the mutated yeast and is grown on the galactose medium. If we assume that all the organisms have originated from same ancestor and some of the genes are homologous in their function then the galactokinase enzyme gene will be expressed in the mutated yeast. Thus, the yeast will be able to produce the enzyme galactokinase that can metabolize the sugar galactose. As a result mutated yeast will be able to survive on galactose medium.
This result indicates that the galactokinase enzyme is homologous gene. Thus, if the yeast gene that encodes the galactokinase is revealed then the human gene composition can also be analyzed.
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