PCR (polymerase chain reaction) — Primary use case: PCR is increasingly the method of choice for which of the following purposes in gene analysis?

Introduction / Context:
PCR is a rapid enzymatic method to amplify specific DNA sequences from tiny amounts of template. It underpins diagnostics, forensics, cloning workflows, and gene screening because it can sensitively and specifically detect target sequences.

Given Data / Assumptions:

• PCR uses primers, thermostable polymerase, and cycling of denaturation, annealing, and extension.
• The question asks about the “method of choice” use among listed options.
• “Alteration” and “sterilization” are not meaningful PCR goals as stated.

Concept / Approach:
PCR excels at detecting whether a particular sequence is present and at what approximate abundance after amplification. PCR products can be analyzed by gel electrophoresis or via quantitative fluorescence in real-time PCR to screen samples for genes, mutations, or pathogens.

Step-by-Step Solution:

Verification / Alternative check:
Clinical diagnostics (e.g., pathogen panels) and research routinely rely on PCR/qPCR for screening due to speed and sensitivity.

Why Other Options Are Wrong:

• Alteration of a gene requires engineering methods; PCR may provide fragments but does not itself edit genomes.
• “Sterilization of a gene” is nonsensical in molecular terms.
• “All of these” is incorrect because the other options are not accurate descriptions.

Common Pitfalls:
Assuming PCR alone equals cloning or editing; PCR is an amplification and detection tool that can be combined with other methods.

Final Answer:
Screening or detecting a gene (amplification-based detection)