Scope of PCR: What does Polymerase Chain Reaction amplify to generate millions of copies?

Introduction / Context:
PCR is a targeted amplification technique. It is designed to selectively copy a defined region between two primers rather than copy the entire genome indiscriminately. Understanding this precision underpins its applications in diagnostics, cloning, and forensics.

Given Data / Assumptions:

• Primers flank the specific DNA region of interest.
• Thermostable DNA polymerase synthesizes new strands.
• The method is specific, not whole-genome amplification (unless specialized variants are used).

Concept / Approach:
Two primers determine boundaries of amplification. With each cycle, only the segment between primers is amplified exponentially. Whole genome amplification requires different strategies (e.g., multiple displacement amplification) and is not standard PCR.

Step-by-Step Solution:
Identify the role of primers in defining a target.Recall exponential doubling of only the bounded segment.Select the option describing a short DNA fragment as the amplified product.

Verification / Alternative check:
Gel electrophoresis after PCR shows a single band of the expected size (if specific), confirming amplification of a discrete fragment rather than genome-wide products.

Why Other Options Are Wrong:

• Entire genome (B): incorrect for standard PCR.
• Both (C): incorrect because (B) is wrong.
• Short chain RNA (D): PCR copies DNA; RT-PCR uses reverse transcription first for RNA.

Common Pitfalls:
Confusing PCR with RT-PCR or WGA; always specify the template and method variant.

Final Answer:
a short fragment of DNA