How do DNA microarrays assess gene expression in cancer research?

DNA microarrays are powerful tools used in cancer research to assess gene expression levels across thousands of genes simultaneously. The technique operates on the principle of hybridization, where labeled cDNA or RNA from a sample is allowed to bind to complementary DNA probes fixed on a microarray. By measuring the intensity of the signals from each probe, researchers can determine which genes are expressed and to what extent.

The correct answer highlights that DNA microarrays evaluate the number of genes that are turned on (expressed) or turned off (not expressed) in a given sample. This information is vital in cancer research because it allows scientists to identify specific gene expression patterns associated with different types of cancer, which can provide insights into disease mechanisms, potential biomarkers for diagnosis, and targets for treatment.

In contrast, analyzing the physical structure of DNA does not directly relate to gene expression. Directly sequencing DNA focuses on determining the nucleotide sequence and does not assess expression levels. Counting cell mitosis rates instead looks at cell proliferation, providing no direct information about gene expression. Thus, option B accurately captures the primary function of DNA microarrays in evaluating gene activity in cancer research.