Question

In the ‘Southern blot’ technique, which of the following reagents is used to detect the presence of a desired DNA fragment?

• A. Ethidium bromide
• B. DNA probe
• C. Silver nitrate
• D. DNase

Answer 1

The Correct Option is B

The 'Southern blot' technique is a method used in molecular biology for detecting specific DNA fragments. This technique is named after its inventor, Edwin Southern, and involves the following key steps:

1. DNA Fragmentation: The DNA is first digested with restriction enzymes to cut it into smaller fragments.
2. Gel Electrophoresis: These fragments are then separated using gel electrophoresis based on their size.
3. Transfer to Membrane: The DNA fragments are then transferred from the gel to a nylon or nitrocellulose membrane, which is more durable and allows for further analysis.
4. Hybridization with Probe: The membrane is incubated with a labeled DNA probe that is complementary to the desired DNA sequence. This is the crucial step for detection.
5. Detection: The probe hybridizes with its complementary DNA sequence on the membrane, and the labeled probe can then be detected using various methods, such as autoradiography or through colorimetric or chemiluminescent signals, depending on the type of label used.

Among the options provided:

• Ethidium bromide: This is a chemical used for staining DNA in gels, but not for probing specific DNA sequences in the blotting process.
• DNA probe: This is indeed used to detect the presence of a specific DNA fragment in the Southern blot. The probe is a short, single-stranded piece of DNA that is labeled with a detectable tag (such as a radioactive or fluorescent label) and is complementary to the target sequence.
• Silver nitrate: Primarily used in staining techniques for proteins and detecting small amounts of nucleic acids, but not used for probing DNA sequences in Southern blotting.
• DNase: An enzyme that digests DNA, which would degrade the sample instead of helping to detect specific fragments.

Conclusion: The correct answer is the DNA probe, as it is specifically designed and labeled to hybridize with and thereby enable the detection of the DNA fragment of interest on the membrane.