Why are radioactive DNA probes used to locate specific DNA fragments?

• A. Because DNA is invisible on gel/membrane
• B. Because they speed up migration through the gel
• C. Because they prevent contamination
• D. Because they increase DNA length

Answer: (A)

DNA fragments on a gel or membrane can’t be seen by eye, so radioactive DNA probes are used to reveal the location of a specific fragment. A radioactive probe is a short DNA sequence that is complementary to the target fragment and is labeled with a radioactive isotope. After the probe binds to its target on the blot, the emitted radiation is detected by autoradiography (exposing X-ray film or a detector), showing exactly where that fragment sits among many. This relies on the specificity of base pairing to identify the correct fragment. The probe’s purpose is not to change how DNA migrates, prevent contamination, or alter DNA length, so those options don’t fit.