A DNA microarray is a small glass or silicon chip that can test the activity of thousands of genes at the same time. Each tiny spot on the chip contains many copies of a DNA probe designed to match one specific gene sequence. In medicine, microarrays help researchers compare healthy and diseased cells, classify tumors, and study how genes respond to drugs.
They matter because many diseases involve patterns of gene activity rather than a change in just one gene.
Key Facts
- A DNA microarray contains thousands of fixed DNA probes arranged in a grid on a small chip.
- Complementary base pairing allows sample DNA or cDNA to bind to matching probes: A pairs with T, and C pairs with G.
- Gene expression is often measured using labeled cDNA made from mRNA in the sample.
- Fluorescence intensity is proportional to the amount of matching genetic material bound at a spot.
- Relative expression can be estimated with fold change = expression in sample A / expression in sample B.
- Microarrays are useful for comparing gene activity patterns in cancer, infection, genetic disorders, and drug response studies.
Vocabulary
- DNA microarray
- A DNA microarray is a chip with thousands of DNA probes used to measure many gene sequences or gene activity levels at once.
- DNA probe
- A DNA probe is a short, known DNA sequence fixed to the chip that binds to a complementary sequence in the sample.
- Gene expression
- Gene expression is the process by which information in a gene is used to make RNA and often a protein.
- Fluorescent label
- A fluorescent label is a glowing chemical tag attached to sample DNA or cDNA so bound molecules can be detected by a scanner.
- Hybridization
- Hybridization is the binding of complementary DNA strands through base pairing.
Common Mistakes to Avoid
- Thinking each colored spot is a whole gene, which is wrong because each spot contains many copies of a short probe sequence that represents part of a gene.
- Assuming brighter fluorescence always means a gene is more important, which is wrong because brightness shows abundance of matching material, not biological importance by itself.
- Forgetting to compare against a control sample, which is wrong because gene expression data usually need a reference to show whether activity is higher or lower.
- Confusing DNA microarrays with DNA sequencing, which is wrong because microarrays test known sequences on a chip while sequencing determines the order of bases in DNA.
Practice Questions
- 1 A microarray spot has a fluorescence intensity of 800 in a tumor sample and 200 in a healthy control. Calculate the fold change in expression for that gene.
- 2 A chip contains 12,000 probe spots. If 15 percent of the spots show strong binding in a sample, how many spots show strong binding?
- 3 A researcher finds that several genes involved in cell division are much more active in a cancer sample than in a normal sample. Explain how a DNA microarray provides evidence for this conclusion.