What is the purpose of a Southern blot in DNA analysis?

• A. To detect and localize specific DNA sequences within a DNA sample after blotting.
• B. To measure RNA expression levels.
• C. To edit genes in living cells.
• D. To determine protein-protein interactions.

Answer: (A)

The main idea being tested is how a Southern blot identifies where a specific DNA sequence is in a sample. In this technique, DNA is first cut with restriction enzymes and separated by size on a gel. The DNA fragments are then transferred to a membrane (blotted). A labeled DNA probe that is complementary to the sequence of interest is added; it will hybridize only to fragments containing that sequence. By detecting the probe, you can see whether the sequence is present and roughly how large the fragment is, which helps infer the location and copy number of the target sequence within the genome.

This is distinct from other analyses: measuring RNA expression levels is done by methods like Northern blot or RT-PCR, which focus on RNA rather than DNA. Gene editing in living cells involves changing the genome (not a blot-based detection). Determining protein-protein interactions is about proteins and uses different techniques such as co-immunoprecipitation or yeast two-hybrid assays.