Hybridization assays: The Southern blot technique relies on what key principle to detect a specific DNA sequence in a complex mixture?

Introduction / Context:
Southern blotting detects specific DNA fragments separated by gel electrophoresis. After transfer to a membrane, a labeled DNA probe hybridizes to its complementary sequence among many fragments, revealing the presence and size of the target. The core principle is Watson–Crick base pairing between the probe and target DNA.

Given Data / Assumptions:

• DNA is digested with restriction enzymes and size-separated on an agarose gel.
• Fragments are denatured and transferred to a membrane.
• A labeled single-stranded DNA probe complementary to the target is used.

Concept / Approach:
Specific detection results from complementary base pairing under defined stringency (temperature and salt). Washes remove nonspecific binding, and the labeled probe is visualized by autoradiography or chemiluminescence. The band position reflects fragment size, aiding mapping and genotyping.

Step-by-Step Solution:

Verification / Alternative check:
Controls with known positive DNA confirm probe hybridization; altering wash stringency verifies sequence specificity. Reprobing after stripping demonstrates reproducibility.

Why Other Options Are Wrong:

• RNA–RNA complementarity describes Northern blotting.
• Protein detection by mass is SDS-PAGE/Western blotting, not Southern.
• Supercoiling differences do not determine Southern blot specificity.

Common Pitfalls:
Poor transfer, insufficient denaturation, or overly low stringency can cause weak signals or background. Optimize hybridization conditions to the probe's melting temperature.

Final Answer:
Similarities between probe DNA and experimental DNA (complementary base pairing)