Polymerase chain reaction (PCR) | Khan Academy

Overview of PCR (Polymerase Chain Reaction)

• PCR is a method used to amplify specific fragments of DNA, producing numerous copies of a particular DNA segment.

Applications of PCR

• Commonly used in:

  • Cloning DNA fragments into plasmids for experiments.

  • Forensics for DNA analysis.

  • Medical diagnostics to detect genes related to conditions.

PCR Process Steps

• Denaturation:

  • Heating the DNA sample to 96°C to separate the double strands.

• Annealing:

  • Cooling down to about 55°C, allowing primers (ordered sequences) to attach to the target region of DNA.

• Extension:

  • Heating to 72°C to allow Taq polymerase, a heat-resistant enzyme, to extend the primers and synthesize new DNA strands using nucleotides.

Important Components of PCR

• Primers:

  • Specific sequences that bind to the ends of the target DNA segment; must be used in surplus to ensure binding.

• Taq Polymerase:

  • A heat-resistant enzyme sourced from Thermus aquaticus, essential for the extension step of the PCR process.

• Nucleotides:

  • Building blocks needed for synthesizing the new DNA strands, must be included in the reaction mix.

Chain Reaction Mechanism

• Each cycle of PCR doubles the amount of targeted DNA fragment:

  • After one cycle, 1 copy becomes 2; continuing cycles lead to exponential growth (e.g., 35 cycles yield about 34 billion copies if perfectly efficient).

Efficiency and Duration

• Typical PCR runs last about 2-3 hours, allowing significant amplification to occur if conditions are optimal.