Q.3

After how many cycles of polymerase chain reaction (PCR), the amplified product of

required length is generated for the first time from a chromosomal DNA template?

(A) 2

(B) 3

(C) 4

(D) 5

PCR Cycles: When Is the First Amplified Product of Required Length Generated?

In polymerase chain reaction (PCR), the amplified product of required length from a chromosomal DNA template first appears after 3 cycles.

PCR Amplification Basics

PCR amplifies specific DNA segments through repeated cycles of denaturation, annealing, and extension, starting from a long chromosomal template. Primers define the target region, and only after sufficient cycles does the exact-length product form between the primers. This process is exponential, but the precise product emerges due to progressive shortening of strands.

Cycle-by-Cycle Breakdown

• Cycle 1: Template strands separate; primers anneal and extend, producing two long strands with partial extensions beyond the required length.

• Cycle 2: New strands form, but products remain longer than the target due to extension from original template ends.

• Cycle 3: Short products from prior cycles serve as templates, yielding the first double-stranded DNA of exact required length bounded by primers.

This milestone at cycle 3 enables detection of the specific amplicon.

Why Not Other Options?

(B) 3	Correct: First exact-length double-stranded product forms here from shortened templates.

(C) 4	By cycle 4, target product dominates (2^4 = 16 molecules), but it first appears at 3.

(D) 5	Cycle 5 yields more (32 molecules), yet not the initial generation.

Key Takeaways for PCR Optimization

Understanding this timing aids in gel electrophoresis analysis and yield predictions. For chromosomal templates, 25-40 total cycles maximize amplification post-initial formation.