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Describe and explain how the polymerase chain reaction (PCR) is used to amplify a DNA fragment - AQA - A-Level Biology - Question 8 - 2021 - Paper 1
Question 8
Describe and explain how the polymerase chain reaction (PCR) is used to amplify a DNA fragment. Figure 9 shows the number of DNA molecules produced using a PCR. Ex... show full transcript
Worked Solution & Example Answer:Describe and explain how the polymerase chain reaction (PCR) is used to amplify a DNA fragment - AQA - A-Level Biology - Question 8 - 2021 - Paper 1
Step 1
Describe and explain how the polymerase chain reaction (PCR) is used to amplify a DNA fragment.
Answer
To amplify a DNA fragment using PCR, several steps are involved:
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Preparation: The process begins with the addition of DNA polymerase, DNA nucleotides, primers, and the DNA fragment to be amplified in a reaction mixture.
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Denaturation: The mixture is heated to approximately 95°C. This high temperature causes the double-stranded DNA to denature, meaning that the hydrogen bonds between the base pairs break, resulting in two separate strands of DNA.
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Annealing: The temperature is then lowered to around 50-65°C, allowing the primers to bind to their complementary sequences on the single-stranded DNA template. This is crucial for the specific amplification of the target DNA fragment.
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Extension: The temperature is increased again to about 70-75°C. At this temperature, the DNA polymerase synthesizes new DNA strands by adding nucleotides complementary to the template strands. This cycle of denaturation, annealing, and extension is repeated multiple times, leading to exponential amplification of the DNA fragment.
Step 2
Explain the shape of the curve in Figure 9.
Answer
The shape of the curve in Figure 9 typically shows an initial phase where the number of DNA molecules is low or remains constant. This is followed by an exponential increase as the PCR process doubles the amount of DNA with each cycle. Eventually, the curve plateaus as the reaction reaches a point where there are no more free primers or nucleotides available. At this plateau phase, the amplification of DNA ceases as the enzyme, DNA polymerase, may also denature, limiting further reactions.