Q.27 In a PCR reaction, with one double stranded DNA of 600 bp , nano gram of DNA produced after 40

cycles of amplification will be ____

1,100,000,000,000 nanograms (1.1 × 10¹² ng) of DNA is theoretically produced after 40 PCR cycles from 1 ng of a 600 bp double-stranded DNA template.​

This calculation assumes ideal 100% efficiency, where the number of DNA molecules doubles each cycle, leading to exponential amplification. In practice, yields plateau earlier due to reagent limitations, but exam questions like this for CSIR NET use the theoretical model.​

PCR Amplification Basics

PCR exponentially amplifies DNA through repeated cycles of denaturation, annealing, and extension. Starting with one double-stranded DNA molecule, after n cycles, the product is 2ⁿ molecules of the target amplicon. For 40 cycles, this yields 2⁴⁰ ≈ 1.1 × 10¹² amplicons per initial template.​

Step-by-Step Calculation

To find nanograms, first compute the molecular weight of the 600 bp dsDNA amplicon (average ~650 g/mol per bp): 600 × 650 = 390,000 g/mol.​

• Assume 1 ng initial template (standard for such problems, as phrasing implies).​

• Initial molecules: (1 × 10⁻⁹ g / 390,000 g/mol) × 6.022 × 10²³ ≈ 1.54 × 10⁹.​

• Final molecules: 1.54 × 10⁹ × 2⁴⁰ ≈ 1.70 × 10²¹.​

• Final mass: (1.70 × 10²¹ × 390,000 / 6.022 × 10²³) × 10⁹ ≈ 1.1 × 10¹² ng.​

In PCR reactions, calculating nanograms of DNA produced after 40 cycles from a 600 bp double-stranded template is key for molecular biology students preparing for CSIR NET. This guide explains the theoretical yield in a PCR reaction with one double-stranded DNA of 600 bp, where 1 ng initial DNA yields massive amplification.​

Why Theoretical Yield Matters

Standard PCR assumes perfect doubling: Final amount = Initial × 2ⁿ, where n=40. For 600 bp dsDNA, molecular weight ≈ 390 kg/mol enables precise ng conversion. Real yields are lower (~10-100 ng/µL), but exams test ideal math.​

Common Exam Options Explained

• Low values (e.g., 1-100 ng): Ignore exponential growth; typical of 1-2 cycles. Incorrect.​

• Moderate (e.g., 1 µg): Plateau phase reality, not 40 cycles. Wrong for theory.​

• High (e.g., 1.1 × 10¹² ng): Matches 2⁴⁰ amplification from 1 ng. Correct.​

• Molar errors: Forget mass conversion or use ssDNA weight (~330 Da/bp). Mistake.​

Master this for CSIR NET: Focus on 2ⁿ molecules, then ng via MW.​