Explain PCR amplification
Describe denaturation, primer annealing and extension by Taq polymerase.
A Level; Cambridge International Biology 9700, 2028–2030. This is curriculum teaching, not medical advice or a claim that a treatment is suitable for an individual. Written responses are self-checked
Before you begin
- DNA stores genetic information; genes can be expressed to produce RNA and protein.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why is Taq polymerase used in PCR?
What you will learn
- Sequence the three PCR stages
- Explain the role of primers and heat-stable Taq polymerase
Build the explanation
PCR amplifies a specified DNA region through repeated cycles. Heating denatures double-stranded DNA into single strands. Cooling allows primers to anneal to complementary sequences flanking the target. At a suitable temperature, Taq DNA polymerase extends the primers by adding nucleotides complementary to the templates.
Use the evidence carefully
Taq polymerase is heat stable, so it remains functional through repeated heating cycles. Primers determine the region copied; PCR amplifies DNA but does not by itself identify a sequence or show a gene’s function.
Pause and explain
Put the PCR steps in order: extension, denaturation, primer annealing.
Worked example
After DNA strands separate, primers bind to sequences around the target. What happens next?
Show the worked solution
- Primers provide starting points.
- Taq polymerase extends them.
Answer Taq polymerase adds complementary nucleotides to extend each primer and make new DNA strands.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name the PCR stage that separates the two DNA strands.
Give me a hint
Identify the relevant material or technology and explain each step.
Compare my reasoning
- The sample is heated.
- This is denaturation.
Denaturation, the heating stage that separates the DNA strands.
Look for these in your work
- Method identified
- Steps linked to function
What determines the start and end regions amplified in PCR?
Give me a hint
Use the stated example to justify a conclusion.
Compare my reasoning
- Primers bind to complementary sites.
- Their positions flank the target region.
The primer binding sites define the DNA region amplified.
Look for these in your work
- Evidence interpreted
- Limits stated
Put the PCR steps in order: extension, denaturation, primer annealing.
Give me a hint
Apply the idea to a new application or trade-off.
Compare my reasoning
- Separate strands first.
- Bind primers, then extend.
Denaturation → primer annealing → extension by Taq polymerase.
Look for these in your work
- Application explained
- Claim appropriately bounded
Common mistakes
- Reversing denaturation and annealing
- Saying Taq polymerase binds primers but does not extend them
What can you explain now?
Why is Taq polymerase used in PCR?
Compare with the explanation
It is heat stable
It remains active through repeated high-temperature cycles.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What occurs during each of the three PCR stages?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.