Which of the following is a step in a PCR?
A) DNA is cooled to a temperature at which the hydrogen bonds holding the strands break.
B) The primers anneal to the complementary sequences on the template DNA.
C) DNA gyrase enzyme relieves the supercoiling ahead of the replication fork.
D) The DNase I synthesizes a new strand of DNA that is complementary to the template DNA.

Question

Quizplus · Accepted Answer

PCR (Polymerase Chain Reaction) is a technique used to amplify a specific segment of DNA. The steps in PCR are as follows:

1. Denaturation: The double-stranded DNA is heated to a high temperature (around 95°C) which breaks the hydrogen bonds between the two strands, separating them into single strands.

2. Annealing: The temperature is lowered (around 50-60°C) and a short segment of single-stranded DNA called a primer is added. The primer binds to the complementary sequence on the single-stranded DNA.

3. Extension: A DNA polymerase enzyme is added along with nucleotides (building blocks of DNA) and the temperature is raised to the optimal temperature for the polymerase. The polymerase uses the primer as a starting point and begins to copy the single-stranded DNA into a double-stranded DNA molecule.

The correct answer is therefore B as it describes the annealing step where the primers bind to the complementary sequences on the template DNA. The other choices, cooling to break hydrogen bonds (A), DNA gyrase enzyme (C), and DNase I (D) are not steps in PCR.

Which of the Following Is a Step in a PCR