1. DNA replication requires DNA-Topoisomerase to remove the supercoiling of DNA that accumulates
at the end of a growing replication fork. You wish to perform a PCR amplification of a gene that
has been provided to you in a 6 kb plasmid vector. Why will you NOT use
topoisomerase in your PCR reaction mix?
(a) Denaturation step in the PCR protocol precludes formation of supercoils ,
(b) Reaction buffer has a pH that denatures DNA and avoids supercoiling
(c) Taq polymerase has innate topoisomerase activity
(d) The 5´ 3´ exonuclease activity of Taq polymerase does not allow supercoiling

Article:

Introduction

Polymerase Chain Reaction (PCR) is a widely used method for amplifying specific DNA sequences. One common question arises when working with plasmid DNA, particularly in the case of long or circular DNA strands: Do we need to include topoisomerase in the PCR reaction to manage supercoiling during DNA replication? In this article, we will explore why topoisomerase is typically not required in PCR amplification and how the process works even without it.

What is Topoisomerase?

Topoisomerase is an enzyme that helps in relieving the supercoiling stress that accumulates in DNA as it is unwound during processes such as DNA replication. Supercoiling occurs when the DNA double helix is over- or under-rotated, causing tension in the molecule. Topoisomerase enzymes, particularly type I and type II, work by making transient cuts in the DNA to relieve this tension and prevent DNA damage.

While topoisomerase plays a critical role during DNA replication in living cells, does it play a similar role during PCR? Let’s break down the process.

How Does PCR Work?

In PCR, DNA is amplified by repeatedly cycling through three key steps:

1. Denaturation: The DNA is heated to 94-98°C, causing the double-stranded DNA to separate into single strands.

2. Annealing: The reaction is cooled to allow short DNA primers to bind to their complementary sequences on the template strand.

3. Extension: The temperature is raised to the optimal working temperature of Taq polymerase (around 75°C), which extends the primers to form a new strand of DNA.

Each of these steps is crucial for amplification, but supercoiling can become an issue, especially in long or circular DNA templates like plasmids.

Why Topoisomerase Is Not Required in PCR

Now, let’s explore why topoisomerase is not required in a typical PCR reaction:

1. Denaturation Step Precludes Supercoiling
   The initial denaturation step in PCR, where the DNA is heated to 94-98°C, causes the double-stranded DNA to separate into single strands. Since supercoiling is a feature of double-stranded DNA, the denaturation process essentially eliminates the supercoiling problem. As a result, supercoiling does not occur during the amplification phase, and topoisomerase is not necessary.

2. Reaction Buffer and pH Levels
   The reaction buffer used in PCR typically contains a pH that helps to denature the DNA. At high temperatures, DNA becomes single-stranded, preventing the formation of supercoils. The high temperature during denaturation also makes the use of topoisomerase unnecessary because the supercoiling is essentially eliminated in the process.

3. Taq Polymerase Does Not Require Topoisomerase
   Taq polymerase, the enzyme commonly used in PCR, has a unique ability to replicate DNA without the need for topoisomerase. It performs the polymerization reaction efficiently at high temperatures without creating significant supercoiling issues. As a result, Taq polymerase does not require assistance from topoisomerase to relieve tension in the DNA strand.

4. Taq Polymerase’s Exonuclease Activity
   Additionally, Taq polymerase has 5’ to 3’ exonuclease activity, which can help remove incorrect nucleotides during DNA synthesis. While this activity does not specifically address supercoiling, it can be advantageous for maintaining the integrity of the amplified product. However, this activity does not eliminate the need for topoisomerase in the PCR reaction.

Answer to the Question

The correct answer to why topoisomerase is not needed in the PCR reaction is:

(a) Denaturation step in the PCR protocol precludes formation of supercoils.

During the denaturation step of PCR, the DNA is unwound and separated into single strands, effectively preventing the supercoiling that would otherwise require topoisomerase to resolve it. This is why topoisomerase is unnecessary for PCR.

Conclusion

In summary, topoisomerase is not needed for PCR amplification because the denaturation step ensures that the DNA is separated into single strands, eliminating the issue of supercoiling. Moreover, Taq polymerase works efficiently under these conditions, making the inclusion of topoisomerase unnecessary. Understanding this helps streamline the PCR process, ensuring that only the essential components are included in the reaction mix.

This knowledge is crucial when working with plasmid vectors or other DNA templates that may otherwise be prone to supercoiling, but in the case of PCR, supercoiling is not a major concern.