Polymerase Chain Reaction (PCR) is a laboratory technique used to amplify a specific segment of DNA, creating millions of copies from a very small initial sample. It is fundamental in genetic research, medical diagnostics, and forensic science.

Key Components Required for PCR:

Template DNA: The DNA sample containing the target sequence to be amplified.
Primers: Short, single-stranded DNA sequences that are complementary to the 3' ends of the target DNA segment. Two primers are used: a forward primer and a reverse primer.
Taq Polymerase: A heat-stable DNA polymerase enzyme (often from the bacterium Thermus aquaticus) that synthesizes new DNA strands.
Deoxyribonucleotide Triphosphates (dNTPs): The building blocks (A, T, C, G) for the new DNA strands.
Buffer Solution: Provides the optimal chemical environment for the reaction, including ${Mg}^{2 +}$ ions which are a cofactor for Taq polymerase.

The Three Main Steps of PCR (One Cycle):

PCR involves repeating a cycle of three temperature-dependent steps 20-40 times. Each cycle doubles the amount of the target DNA segment.

Step 1: Denaturation

The reaction mixture is heated to a high temperature (94-98°C) for 15-30 seconds. This breaks the hydrogen bonds between the two strands of the double-stranded DNA template, separating them into two single strands.

Step 2: Annealing

The temperature is lowered to 50-65°C for 20-40 seconds. This allows the primers to bind (anneal) to their complementary sequences on each of the single-stranded DNA templates. The temperature is critical for specific binding.

Step 3: Extension/Elongation

The temperature is raised to the optimum for Taq polymerase (72°C) for 30-60 seconds. Starting from the primers, Taq polymerase synthesizes new DNA strands by adding dNTPs to the 3' end of each primer, creating two new double-stranded DNA molecules.

After the first cycle, there are 2 copies of the target DNA. After the second cycle, there are 4 copies. This exponential amplification continues, resulting in over a billion copies after 30 cycles.

Visualizing the Process:

While I cannot draw a diagram, imagine a test tube in a machine (a thermocycler) that rapidly changes temperature. The process is often depicted as a flow chart:

Cycle 1: 1 dsDNA → 2 dsDNA
Cycle 2: 2 dsDNA → 4 dsDNA
Cycle 3: 4 dsDNA → 8 dsDNA
... and so on, until the desired amplification is achieved.

Related Topics & Formulae

Related Theory: PCR is a form of in vitro DNA replication. Its invention revolutionized molecular biology by allowing scientists to work with tiny amounts of DNA. Key to its success is the use of a thermostable polymerase, which survives the repeated high temperatures of the denaturation step.

Formula for DNA Amplification: The number of double-stranded DNA molecules produced after $n$ cycles can be calculated by the formula: $2^{n}$ For example, after 30 cycles: $2^{30} = 1,073,741,824$ copies.