Edexcel A Level (IAL) Biology -6.17 Polymerase Chain Reaction- Study Notes- New Syllabus

PCR: Polymerase Chain Reaction

🌱 Introduction

PCR is a lab method used to make millions of copies of a specific DNA fragment. It is useful when only a tiny amount of DNA is available, such as in forensic samples, medical tests or genetic studies.

🟣 What PCR Needs (Basic Requirements)

Template DNA

• The DNA that contains the target sequence to be copied.

Primers

• Short single-stranded DNA pieces that bind to each end of the target sequence.

DNA nucleotides (A, T, C, G)

• Building blocks used to make new DNA strands.

Taq DNA polymerase

• A heat-tolerant enzyme from Thermus aquaticus.
• Remains active even at high temperatures.

Thermocycler

• A machine that repeatedly changes temperature for each stage of PCR.

🔵 The Three Main Stages of PCR

1. Denaturation (about 95°C)

• Double-stranded DNA is heated.
• Hydrogen bonds break.
• DNA separates into two single strands.

2. Annealing (about 55°C)

• Temperature is lowered.
• Primers bind to complementary sequences on each target DNA end.
• This step decides which region will be copied.

3. Extension (about 72°C)

• Taq polymerase adds nucleotides to primers.
• New complementary strands form.
• Each cycle doubles the target DNA.

🟢 How PCR Achieves Amplification

• One cycle doubles DNA quantity.
• After n cycles, amount = \(2^n\) times the original.
• 30–40 cycles produce millions of copies.
• Makes PCR extremely sensitive and fast.

🟠 Uses of PCR (Why It’s Important)

• Detecting infections (e.g., viral DNA or RNA after reverse transcription).
• Forensic DNA analysis.
• Genetic testing and diagnostics.
• Research: cloning genes, preparing DNA for sequencing.
• Identifying GM organisms or pathogens.

📋 Summary Table