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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.
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
Step 1: Requires DNA fragment, DNA polymerase, nucleotides, and primers
Answer
To initiate the PCR process, a DNA fragment that needs to be amplified is mixed with essential components including DNA polymerase (commonly Taq polymerase), nucleotides, and primers. The primers are short sequences that provide a starting point for DNA synthesis.
Step 2
Step 2: Heating to 95°C to separate strands
Answer
The mixture is heated to 95°C, which causes the hydrogen bonds between the strands of DNA to break, thereby denaturing the DNA and separating the two strands. This step is crucial as it allows access to the template DNA.
Step 3
Step 3: Cooling for primer binding
Answer
The temperature is then reduced to around 50-65°C, allowing the primers to bind (anneal) to their complementary sequences on the single-stranded DNA templates. This step is vital for ensuring that DNA synthesis starts at the correct location.
Step 4
Step 4: DNA polymerase synthesis
Answer
Finally, the temperature is increased to approximately 70-75°C, which is the optimum temperature for DNA polymerase activity. The enzyme extends the primers by adding nucleotides, synthesizing new strands of DNA. This cycle of heating and cooling is repeated multiple times to exponentially amplify the DNA fragment.