What is the PCR Technique?
Polymerase Chain Reaction (PCR) is a laboratory technique that exponentially amplifies a specific segment of DNA, turning a tiny sample into millions or billions of copies within hours. It relies on repeated heating and cooling cycles and a heat-stable DNA polymerase.
PCR is a technique that exponentially copies a target DNA sequence through repeated cycles of denaturation, annealing, and extension, using a heat-stable DNA polymerase such as Taq.
- 1.Denaturation (~95°C) — Heat separates the double-stranded DNA template into two single strands.
- 2.Annealing (~50-65°C) — Short primers bind to complementary sequences flanking the target region on each single strand.
- 3.Extension (~72°C) — Taq polymerase extends each primer, synthesizing a new complementary strand and doubling the target DNA.
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Step-by-step worked examples
Starting with 5 copies of target DNA, how many copies exist after 25 cycles of PCR (assuming 100% efficiency)?
N = N₀ × 2ⁿ N = 5 × 2²⁵ N = 5 × 33,554,432 = 167,772,160 copies
A sample starts with 1 copy of DNA and undergoes 30 cycles. How many copies result?
N = N₀ × 2ⁿ N = 1 × 2³⁰ N = 1,073,741,824 copies (about 1 billion)
A PCR reaction reaches 4,096 copies after 12 cycles. How many starting copies (N₀) were there?
N = N₀ × 2ⁿ 4096 = N₀ × 2¹² 4096 = N₀ × 4096 N₀ = 1 copy
Flashcards
Quick quiz
Q1.What does PCR stand for?
Q2.Which step of a PCR cycle separates the DNA double helix?
Q3.Starting with 2 copies of DNA, how many copies exist after 10 cycles (100% efficiency)?
Q4.Which enzyme is essential to PCR because of its heat stability?
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Common mistakes
Thinking PCR creates new DNA sequences. — Correct: PCR only amplifies (copies) an existing DNA sequence — it doesn't add new genetic information.
Assuming every PCR cycle doubles DNA exactly, regardless of conditions. — Correct: Real reactions have less than 100% efficiency, especially in later cycles (the plateau effect), so actual yield is lower than the theoretical 2ⁿ.
Believing PCR requires living cells. — Correct: PCR is done entirely in vitro, in a test tube with a thermocycler — no living cells are involved.
Treating primers as optional in a PCR reaction. — Correct: Primers are essential — they define the start and end of the amplified region, and DNA polymerase needs a primer to begin synthesis.
FAQ
What is PCR (Polymerase Chain Reaction)?
PCR is a laboratory technique that exponentially amplifies a specific DNA sequence using repeated cycles of heating and cooling with a heat-stable DNA polymerase.
What is the PCR technique formula?
N = N₀ × 2ⁿ, where N₀ is the starting number of DNA copies and n is the number of amplification cycles, assuming 100% efficiency.
What are examples of PCR technique uses?
COVID-19 diagnostic testing, DNA fingerprinting in forensics, and paternity testing are common real-world applications of PCR.
How do you calculate DNA copies produced by PCR?
Multiply the starting number of DNA copies by 2 raised to the power of the number of cycles: N = N₀ × 2ⁿ.