Name: pcr cycle for dna amplification
Uploaded: 2026-08-26T08:19:52.163Z
Duration: 2 min 7 s
Channel: JoVE

The goal of PCR, or Polymerase Chain Reaction, is to amplify a specific genetic sequence. In this example, a gene of interest is amplified from purified template DNA.To perform the amplification, a thermostable DNA polymerase such as Taq polymerase is required to synthesize new DNA strands. It remains functional during the repeated high-temperature steps of PCR.Primers, which are short single-stranded DNA sequences complementary to regions flanking the gene of interest, are also required. In addition, deoxynucleoside triphosphates, or dNTPs, act as the building blocks of DNA synthesis.The first step in PCR is heating the reaction mixture to denature the double-stranded DNA into single strands.Next, the reaction is cooled to allow the primers to anneal to their complementary target sequences. The reaction is then heated to the extension temperature that is optimal for DNA polymerase function.The polymerase recognizes the primer-template complex and extends the primers by incorporating dNTPs to synthesize new DNA strands.The reaction then undergoes repeated cycles of denaturation, primer annealing, and extension.Following the third cycle, eight copies of the target sequence are present. After the fourth cycle, 16 copies are generated. After the fifth cycle, 32 copies are present.With each cycle, the amount of DNA increases exponentially. After approximately 30 cycles, more than one billion copies of the target sequence can be produced under ideal reaction conditions.

The polymerase chain reaction, or PCR, is a widely used technique for copying segments of DNA. Due to exponential amplification, PCR can produce millions or billions of DNA copies within just a few hours. In a PCR reaction, a heat-resistant DNA polymerase enzyme amplifies the original DNA through a series of temperature changes inside an automated machine called a thermocycler.<h4>PCR is a Versatile Method that Revolutionized Molecular Biology</h4>Kary Mullis developed PCR in 1983, for which he was awarded the 1993 Nobel Prize in Chemistry. Being a relatively fast, inexpensive, and precise way of copying a DNA sequence, PCR became an invaluable tool for numerous applications, including molecular cloning, gene mutagenesis, pathogen detection, gene expression analysis, DNA quantitation and sequencing, and genetic disease diagnosis.<h4>The PCR Reaction Cycle</h4>PCR mimics the natural DNA replication process that occurs in cells. The reaction mixture includes a template DNA sequence to be copied, a pair of short DNA molecules called primers, free DNA building blocks called deoxynucleotide triphosphates (dNTPs), and a specialized DNA polymerase enzyme.PCR involves a series of steps at high temperatures, requiring a DNA polymerase enzyme that is functional at such temperatures. The most commonly used DNA polymerase is Taq polymerase, named after Thermus aquaticus, the bacterium from which the polymerase was initially isolated. DNA polymerase is unable to synthesize a DNA molecule from scratch, or de novo. Instead, DNA polymerase adds to short DNA molecules, called primers, which bind to the DNA template through complementary base pairing. The primers provide a free 3&#x2019; hydroxyl group to which DNA polymerase can attach new dNTPs. There are four types of dNTPs in a PCR, one for each nucleotide in the DNA molecule: dATP, dCTP, dGTP, and dTTP.Each PCR cycle consists of three steps: Denaturation, Annealing, and DNA Synthesis.<ol><li>Denaturation. The PCR cycle is initiated by heating the reaction mixture to a high temperature, causing separation of the DNA double helix into two strands. This &#x201C;melting&#x201D; process usually occurs at a temperature of 90&#xB0;C&#xAD;&#xAD;&#x2013;100&#xB0;C.</li><li>Annealing. The reaction mixture is quickly cooled (usually to 50&#xB0;C&#x2013;65&#xB0;C), allowing the two primers to bind to their complementary sequences on the template DNA strands.</li><li>DNA Synthesis (Primer Extension). The reaction mixture is heated again, this time to a temperature (usually 60&#xB0;C&#x2013;75&#xB0;C) that allows DNA polymerase to extend the primers by adding dNTPs that pair with the bases in the template strand.</li></ol>A typical PCR involves 20-40 repeated cycles of these three steps, occurring in the thermocycler. Since the number of DNA molecules is doubled in each cycle, the DNA is amplified exponentially.<h4>Limitations of PCR</h4>If the scientist wants to amplify a specific stretch of the genome, the scientist must know at least part of the target DNA sequence to design appropriate primers. Another potential issue is the nonspecific annealing of primers to partially similar DNA sequences, leading to amplification of non-target DNA. This issue can be controlled by optimizing the reaction conditions. Being a highly sensitive detection method, PCR is also vulnerable to contamination, and even trace amounts of contaminating DNA can cause misleading results. The DNA polymerases used in PCR can be prone to errors. If a mutation happens within the first few cycles, most of the amplified DNA will carry the mutation.

Overview

The polymerase chain reaction, or PCR, is a widely used technique for copying segments of DNA. Due to exponential amplification, PCR can produce ...

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pcr cycle for dna amplification

PCR is a laboratory method for making many copies of a DNA segment. It can turn a small amount of DNA into millions or even billions of copies in just a few hours. The process uses a heat-resistant DNA polymerase inside a machine called a thermocycler.

PCR was developed by Kary Mullis in 1983, and he received the 1993 Nobel Prize in Chemistry for it. The method is fast, relatively inexpensive, and precise. It has become useful for molecular cloning, gene mutagenesis, pathogen detection, gene expression analysis, DNA quantitation, sequencing, and genetic disease diagnosis.

PCR copies DNA in a way that resembles natural DNA replication. The reaction mixture includes template DNA, short DNA primers, free DNA building blocks called deoxynucleotide triphosphates, or dNTPs, and a special DNA polymerase. The most common enzyme is Taq polymerase, which comes from the bacterium Thermus aquaticus.

DNA polymerase cannot start a DNA strand from scratch. It needs primers, which are short DNA molecules that bind to the template by complementary base pairing. The primers provide a free 3' hydroxyl group so the polymerase can add new dNTPs. The four dNTPs used in PCR are dATP, dCTP, dGTP, and dTTP.

Each PCR cycle has three steps: denaturation, annealing, and DNA synthesis. During denaturation, the mixture is heated to about 90°C to 100°C, which separates the double helix into two strands. During annealing, the mixture cools to about 50°C to 65°C so the primers can bind to their matching sequences.

During DNA synthesis, also called primer extension, the mixture is heated again to about 60°C to 75°C. At this stage, DNA polymerase extends the primers by adding dNTPs that match the template strand. A typical PCR uses 20 to 40 cycles in the thermocycler, and the DNA amount doubles each cycle, so amplification is exponential.

PCR has limits that scientists must watch for. To copy a specific genome region, the target sequence must be known well enough to design the right primers. Primers can also bind to similar non-target DNA, which can create unwanted products, but reaction conditions can be adjusted to reduce this problem.

Because PCR is very sensitive, contamination can cause misleading results even when only trace amounts of outside DNA are present. The DNA polymerases used in PCR can also make mistakes. If a mutation happens early in the cycle, most of the amplified DNA will carry that mutation.

PCR Cycle for DNA Amplification

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PCR is a laboratory method for making many copies of a DNA segment. It can turn a small amount of DNA into millions or even billions of copies in just a few hours. The process uses a heat-resistant DNA polymerase inside a machine called a thermocycler. PCR was developed by Kary Mullis in 1983, and he received the 1993 Nobel Prize in Chemistry for it. The method is fast, relatively inexpensive, and precise. It has become useful for molecular cloning, gene mutagenesis, pathogen detection, gene...

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key terms and concepts

Learn how PCR amplifies DNA through denaturation, annealing, and extension, plus the role of primers, Taq polymerase, and common PCR limits.

PCR Cycle for DNA Amplification | JoVE K12

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PCR Cycle for DNA Amplification

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