What do DNA microarrays measure in relation to gene expression?

DNA microarrays are powerful tools used to measure gene expression levels across many genes simultaneously. They work by hybridizing complementary DNA or RNA sequences to a fixed array of oligonucleotides, which represent different genes. The primary focus of DNA microarrays is to quantify the amount of messenger RNA (mRNA) present in a sample, which directly correlates to gene expression levels.

When a gene is expressed, the corresponding mRNA is synthesized and can be labeled and hybridized to the microarray. The intensity of the signal from each spot on the microarray reflects the quantity of that mRNA, allowing researchers to determine how much of each gene is being expressed in the sample at that particular time. This makes it a valuable tool for studying gene activity, understanding cellular responses, and analyzing differences between various biological states, diseases, or treatments.

The other choices do not accurately describe the capabilities of DNA microarrays; they are not used to measure cell counts, protein levels, or specific DNA sequence variations. Instead, the technique specifically focuses on mRNA, making option B the accurate description of what DNA microarrays measure regarding gene expression.