The polymerase chain reaction (PCR) is a process used to help solve crimes - Scottish Highers Human Biology - Question 2 - 2019

In stage 3, the temperature allows replication of DNA, facilitating the action of DNA polymerase in adding nucleotides to the primer.

To find out how long it takes for at least 2000 copies, we start with one original copy. Each PCR cycle doubles the number of copies:

• After 1 cycle: 2 copies
• After 2 cycles: 4 copies
• After 3 cycles: 8 copies
• After 4 cycles: 16 copies
• After 5 cycles: 32 copies
• After 6 cycles: 64 copies
• After 7 cycles: 128 copies
• After 8 cycles: 256 copies
• After 9 cycles: 512 copies
• After 10 cycles: 1024 copies
• After 11 cycles: 2048 copies

It takes 11 cycles to produce at least 2000 copies. Since each cycle takes 3 seconds, the total time required is:

11 cycles × 3 seconds/cycle = 33 seconds.

In stage 2, primers bind to the ends of the DNA strands. They anneal to the complementary sequence, providing a starting point for DNA synthesis.

In stage 3, primers facilitate the action of DNA polymerase, which adds complementary nucleotides to the DNA, thereby replicating the DNA.

PCR can now be used to solve crimes from 1980 because it enables the amplification of small amounts of DNA from minute samples, such as a small blood spot, making it possible to analyze and match DNA from crime scenes.