You are working in a human genetics laboratory that studies causes and treatments for eye cataracts in newborns.

You are 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 an enzyme called galactokinase, but the human gene that encodes this enzyme has not yet been identified. At a talk by a visiting scientist, you learn about a strain of bakers’ yeast that contains a mutation called gal1– in its galactokinase gene. Because this gene is needed to metabolize galactose, the mutant 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 if the human galactokinase gene can function in yeast. Since you have an optimistic temperament, you decide to pursue this line 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. You then transform the pool of plasmids into gal1– (gal1 minus) yeast cells so that each cell receives a single plasmid. What do you think will happen when you spread the plasmidcontaining cells on Petri plates that contain galactose as a carbon source? How can this approach help you find the human gene encoding galactokinase?

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