**Question:** Mutation that completely disrupts the function of the gene used in which of the following techniques?

A. Polymerase Chain Reaction (PCR)
B. Southern Blotting
C. DNA Ligase
D. PCR Cloning

**Correct Answer: B. Southern Blotting**

**Core Concept:** Southern Blotting is a molecular biology technique used to analyze the size, copy number, and structure of DNA fragments contained in a DNA sample.

**Why the Correct Answer is Right:** Southern Blotting is used to analyze DNA samples after they have been restricted by an enzyme that cuts the DNA at specific sites. When this occurs, the DNA fragments are separated based on their size using techniques like agarose gel electrophoresis. DNA fragments are then blotted onto a membrane where they can be probed with a labeled DNA or RNA molecule to detect specific DNA sequences.

**Why Each Wrong Option is Incorrect:**

A. Polymerase Chain Reaction (PCR): PCR is a technique used to amplify specific DNA sequences. It does not involve analyzing the size, copy number, or structure of DNA fragments.

B. Southern Blotting: Although PCR is used to produce DNA fragments for Southern Blotting, it is not the technique that directly analyzes the DNA fragments. Southern Blotting is the technique that performs this analysis.

C. DNA Ligase: DNA ligase is an enzyme used in molecular biology to join DNA fragments with overhangs, but it is not the technique used to analyze the size, copy number, or structure of DNA fragments.

D. PCR Cloning: PCR cloning is a technique used to clone DNA fragments amplified by PCR, but it does not directly analyze the DNA fragments themselves.

**Clinical Pearl:**

Southern Blotting is an essential tool in molecular biology for studying DNA fragments, as it allows for the detection and analysis of specific DNA sequences after digestion and separation of DNA in agarose gels. This technique aids in various applications, including:

1. Determining the number and size of DNA fragments in a genomic DNA sample.
2. Identifying the presence or absence of specific DNA sequences in a sample.
3. Confirming that a DNA fragment has been successfully ligated into a plasmid vector during cloning experiments.

By understanding the role of Southern Blotting, medical professionals can effectively utilize this technique in their research and clinical work, contributing to a deeper understanding of genetic disorders, gene expression studies, and molecular diagnostics.