Question

You are interpreting data on a DNA chip or microarray. You expose the chip to a mixture of two cDNA populations: one from cells that were not treated with a glucocorticoid hormone (untreated controls; labeled with a red fluorescent dye) and a population from cells that were treated with glucocorticoid hormones (glucocorticoid-treated; labeled with green fluorescent dye). You look at a spot on the chip representing the gene for phosphoenolase, a gene that is turned off by glucocorticoid treatment, but is expressed in control, untreated cells. What color should the spot representing the phosphoenolase gene be

Answer 1

No color (the spot is not labeled)

Explanation:

A DNA chip is a collection of short DNA probes (DNA spots) that are attached to a solid surface, which is used to measure the expression level of target genes in a sample. When a cDNA population is washed over the DNA chip, those fragments with sequence complementarity to the probes hybridize and thus they can be detected by the fluorescence emitted from the spot area. In this case, the spot containing a probe for the phosphoenolase gene is not labeled, and thereby this spot will not emit fluorescence.