When DNA fragments are produced by an investigator, which test is used to determine if the desired fragment is present?

• A. Western blot
• B. Northern blot
• C. Southern blot
• D. Eastern blot

Answer: (C)

The test being used is designed to detect a specific DNA sequence among many DNA pieces, so it uses a DNA probe that is complementary to the fragment of interest. After the DNA is separated by size, it’s transferred to a membrane and then exposed to a labeled DNA probe. The probe will hybridize only to the matching sequence, and the signal reveals whether the desired DNA fragment is present. This approach is known as a Southern blot.

Other blot types are used for different molecules: Western blot detects proteins using antibodies, Northern blot detects RNA with a nucleic acid probe, and Eastern blot is associated with other biomolecules and is not used for identifying DNA fragments. So for confirming a DNA fragment’s presence, Southern blot is the appropriate choice.