PCR: 聚合酶链式反应
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Q1: What are the three main temperature steps in a PCR cycle?
PCR uses three distinct temperature steps called denaturation, annealing, and extension. Denaturation at 95°C breaks hydrogen bonds between DNA base pairs, creating single-stranded DNA. Annealing at 50-65°C allows primers to bind to the template. Extension at 72°C enables DNA polymerase to synthesize new complementary strands by adding nucleotides.
Q2: Why is Taq polymerase used in PCR reactions?
Taq polymerase is a thermostable enzyme isolated from the bacterium Thermus aquaticus, which lives in hot springs. It can withstand temperatures exceeding 90°C, making it ideal for PCR because it remains active throughout repeated heating and cooling cycles without denaturing, unlike standard DNA polymerases.
Q3: What are the key requirements for designing PCR primers?
Effective primers must be 15-30 base pairs long with approximately 50% guanine and cytosine content. Both primers should have melting temperatures above 50°C and within 1-2 degrees of each other. Primers must be complementary to flanking regions of your target sequence, avoid forming primer dimers, and lack secondary structure from self-annealing.
Q4: How does PCR amplify DNA exponentially?
After each cycle, primers bind to newly synthesized single-stranded DNA. The amplicon forms when polymerase extends from the forward primer on a strand generated by reverse primer amplification, or vice versa. Once created, this amplicon serves as template in subsequent cycles, doubling the target DNA with each cycle and producing exponential amplification after 20-40 cycles.
Q5: What essential reagents must be included in a PCR reaction?
A complete PCR reaction requires DNA template, forward and reverse primers, Taq polymerase, dNTPs (nucleotide building blocks), reaction buffer containing magnesium and other ions, and PCR-grade water. A typical 50-microliter reaction includes specific concentrations of each component. The buffer maintains pH and provides cofactors necessary for polymerase function and reaction stability.
Q6: How do you verify successful PCR amplification?
After PCR completes, verify the product using gel electrophoresis concept procedure and applications to visualize DNA fragments. Successful amplification appears as a band at the correct base pair size matching your expected amplicon. Running negative controls without template and positive controls with known template helps confirm specificity and detect contamination in your reactions.
Q7: What is a PCR master mix and when should you use it?
A PCR master mix is a large-volume mixture of all shared reagents distributed into multiple reaction tubes, useful when amplifying the same product from numerous different templates. This approach reduces setup time and minimizes pipetting errors across many reactions. Each tube receives the master mix plus its specific DNA template, streamlining high-throughput PCR workflows.