PCR (Polymerase Chain Reaction)
PCR is a molecular biology technique used to amplify specific DNA sequences exponentially through repeated cycles of denaturation, annealing, and extension. Developed by Kary Mullis in 1983, it allows scientists to produce millions of copies of a target DNA segment from even minute quantities of template DNA. PCR is foundational to genetic research, diagnostics (including COVID-19 testing), forensic analysis, and cloning applications.
Key Formula
N = N0 × 2^n
LaTeX: N = N_0 \times 2^n
| Symbol | Meaning | Unit |
|---|---|---|
| N | Number of DNA copies after amplification | copies |
| N_0 | Initial number of template DNA molecules | copies |
| n | Number of PCR cycles | dimensionless |
Worked Example
Problem
A PCR reaction starts with 10 template DNA molecules. If the reaction runs for 30 cycles with 100% efficiency, how many DNA copies will be produced?
Solution
Step 1: Identify values. N0 = 10 molecules, n = 30 cycles. Step 2: Apply the PCR amplification formula: N = N0 × 2^n. Step 3: Calculate 2^30 = 1,073,741,824. Step 4: Multiply: N = 10 × 1,073,741,824 = 10,737,418,240.
Answer
Approximately 1.07 × 10^10 copies (about 10.7 billion DNA molecules)
PCR Cycle Stages and Their Conditions
| Stage | Temperature (°C) | Duration | Purpose |
|---|---|---|---|
| Denaturation | 94–98 | 20–30 seconds | Separate double-stranded DNA into single strands |
| Annealing | 50–65 | 20–40 seconds | Primers bind to complementary sequences on template |
| Extension | 72 | 1 minute per kb | Taq polymerase synthesises new DNA strand |
| Initial Denaturation | 95–98 | 2–5 minutes | Fully denature template before cycling begins |
| Final Extension | 72 | 5–10 minutes | Complete any partial extension products |
Interactive Tools
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Related Terms
DNA Replication
DNA replication is the biological process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules, each containing one original and one newly synthesized strand. This semi-conservative process is essential for cell division, ensuring that each daughter cell receives a complete copy of the genetic information. It is carried out by a complex of enzymes including DNA polymerase, helicase, primase, and ligase, and occurs during the S phase of the cell cycle.
Gel Electrophoresis
Gel electrophoresis is a laboratory technique used to separate macromolecules — primarily DNA, RNA, or proteins — by size and charge as they migrate through a porous gel matrix under the influence of an electric field. Negatively charged nucleic acids migrate toward the positive electrode (anode), with smaller fragments travelling faster and further than larger ones, producing a pattern of bands that can be visualised by staining with ethidium bromide or SYBR Green and exposing to UV light. Gel electrophoresis is one of the most widely used techniques in molecular biology, underpinning applications from forensic DNA profiling and paternity testing to restriction mapping, PCR product verification, and Southern blotting.
CRISPR-Cas9
CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats – CRISPR-associated protein 9) is a revolutionary gene-editing technology that uses a guide RNA to direct the Cas9 endonuclease to a specific DNA sequence, where it makes a precise double-strand cut. Originally discovered as a bacterial adaptive immune system, it was adapted for genome editing by Jennifer Doudna and Emmanuelle Charpentier in 2012, earning them the Nobel Prize in Chemistry in 2020. CRISPR-Cas9 enables targeted gene knockout, correction, and insertion across virtually any organism.
PCR stands for Polymerase Chain Reaction. "Polymerase" from Greek polys (many) + meros (part), referring to the enzyme that assembles nucleotides. "Chain" refers to the cascading exponential amplification. Coined by Kary Mullis in 1983 at Cetus Corporation, for which he received the Nobel Prize in Chemistry in 1993.