Question

Which of the following is used as blocking agent in hybridization analysis?

• A. Salmon sperm DNA
• B. SSPE
• C. NaOH
• D. NaCl

Hint:

Blocking is analogous to "priming a wall" before painting. Without the primer (blocking agent), the paint (probe) might soak into the wall (membrane) unevenly, ruining the final image.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Hybridization analysis, such as Southern or Northern blotting, involves binding a labeled nucleic acid probe to a target sequence immobilized on a membrane (nitrocellulose or nylon). Blocking is a critical step performed before the addition of the probe to prevent non-specific binding of the probe to the membrane surface itself, which would otherwise result in a high level of background noise and poor signal-to-noise ratio.

Step 2: Detailed Explanation:
The membranes used in blotting techniques have a high affinity for all nucleic acids. If a labeled probe is added directly, it will bind not only to the complementary target sequences but also to any vacant sites on the membrane. To prevent this, "blocking agents" or "carrier DNA" are used to saturate these unoccupied binding sites. Salmon sperm DNA is the most widely used blocking agent in molecular biology for this purpose. Before use, it is typically sheared into smaller fragments and denatured by boiling to ensure it effectively coats the membrane. It acts as a generic competitor that occupies non-specific binding sites, ensuring that the labeled probe only interacts with its specific complementary sequence on the immobilized DNA or RNA.
In contrast, SSPE (Sodium Chloride-Sodium Phosphate-EDTA) is a hybridization buffer that maintains the pH and ionic strength. NaOH is used to denature double-stranded DNA into single strands, and NaCl provides the necessary salt concentration to stabilize duplexes during the wash steps.

Step 3: Final Answer:
Salmon sperm DNA is used as a blocking agent to reduce non-specific probe binding in hybridization experiments.