135. Which of the following enzymes adds an extra Adenine at the 3′ end of a DNAstrand?
1. Klenow DNA polymerase
2. Taq DNA polymerase
3. Reverse transcriptase
4. Topoisomerase

Question:

Which of the following enzymes adds an extra Adenine at the 3′ end of a DNA strand?

1. Klenow DNA polymerase

2. Taq DNA polymerase

3. Reverse transcriptase

4. Topoisomerase

Correct Answer:

2. Taq DNA polymerase

Detailed Explanation:

In molecular biology, the addition of an extra nucleotide to the 3′ end of a DNA strand is a significant process, especially in techniques like Polymerase Chain Reaction (PCR). The enzyme responsible for this is Taq DNA polymerase, which is commonly used in PCR amplifications. Here’s a breakdown of how this enzyme works and the other enzymes listed:

Taq DNA Polymerase:

• Taq DNA polymerase is a thermostable enzyme that comes from the bacterium Thermus aquaticus, which thrives in high-temperature environments like hot springs.

• One of the characteristics of Taq DNA polymerase is its ability to add an extra Adenine (A) residue to the 3′ end of a DNA strand. This happens because Taq DNA polymerase exhibits terminal transferase activity, which allows it to add extra nucleotides at the 3′ end of a DNA strand without a template.

• This property is particularly useful in TA cloning and other molecular techniques that require the addition of an extra A at the 3′ end of a PCR product, making it easier to insert the amplified DNA into vectors that have complementary T-overhangs.

Klenow DNA Polymerase:

• Klenow DNA polymerase is a fragment of the DNA polymerase I enzyme from E. coli. While Klenow polymerase is highly accurate and can synthesize DNA in both directions, it does not add extra nucleotides to the 3′ end of a DNA strand. Instead, it typically performs 5′ to 3′ polymerization with high fidelity but without the terminal transferase activity seen in Taq polymerase.

Reverse Transcriptase:

• Reverse transcriptase is an enzyme used to transcribe RNA into complementary DNA (cDNA). It is essential in retroviral replication and reverse transcription, but it does not add extra Adenine residues to the 3′ end of DNA. Instead, it synthesizes a complementary DNA strand based on the RNA template.

Topoisomerase:

• Topoisomerase is an enzyme involved in the relaxation and untangling of DNA. It helps manage DNA supercoiling during replication and transcription but does not add nucleotides to the DNA strand.

Conclusion:

The correct answer is Taq DNA polymerase. This enzyme is well-known for adding an extra Adenine residue to the 3′ ends of DNA strands, making it a valuable tool in various molecular biology applications, especially in PCR and cloning.