1.2b Polymerase Chain Reaction (PCR)

Question 1

What is PCR?

Answer 1

A technique used in the lab to target specific sequences of DNA and artificially amplify them (produce billions of copies of them)

Question 2

How are primers involved in PCR?

Answer 2

Each primer is a short strand of nucleotides which are complementary to a specific target sequence at the two ends of the region on the DNA strand to be replicated.

Question 3

What does PCR require?

Answer 3

• DNA template strand
• A specific primer
• Heat tolerant DNA polymerase
• Supply of the four DNA nucleotides

Question 4

In PCR, why is the DNA strand first heated to 92-98°C?

Answer 4

To break the weak hydrogen bonds between base pairs and separate the two DNA strands.

Question 5

In PCR, why is the solution then cooled to 50-65°C?

Answer 5

To allow complementary primers to bind to their specific target sequence.

Question 6

In PCR, after cooling, why is the solution then reheated to 70-80°C?

Answer 6

To allow heat tolerant DNA polymerase to synthesise new DNA strands using free DNA nucleotides

Question 7

Why must heat tolerant DNA polymerase be used?

Answer 7

So the enzyme does not denature due to the extreme high temperatures.

Question 8

Where is PCR carried out?

Answer 8

In vitro (outside the body of the organism)

Question 9

Before the process of PCR, there is one DNA molecule. After seven cycles of PCR, how many DNA molecules will there be?

Answer 9

Cycles - DNA molecules
0 - 1
1 - 2
2 - 4 
3 - 8
4 - 16
5 - 32
6 - 64
7 - 128 = 128 DNA molecules

Question 10

What are practical applications of PCR?

Answer 10

• Forensics
• Diagnosis of Genetic disorders
• Early detection of infection
• Archaeobiology