Following statements were made about supercoiling of DNA:

A. DNA supercoiling acts as a regulator of gene expression, but not for genome organization.
B. Circular DNAs found in mitochondria, viruses and bacteria are invariably negatively supercoiled.
C. A moving RNA polymerase generates positive superhelical tension in the DNA in front of it, and negative helical tension behind it.
D. Human topoisomerase I cuts both the strands of supercoiled DNA to undergo a controlled rotation to relax the supercoiled DNA.
E. Human topoisomerase II makes a transient break in single strands of a DNA duplex which rotates around a phosphodiester bond in the intact strand to relax the supercoiled DNA.

Which one of the following options is a combination of all correct statements?

Correct option is D

Explanation:

• B is correct: Circular DNA found in mitochondria, viruses, and bacteria is indeed negatively supercoiled. This is the common feature of circular DNA in these organisms.

• C is correct: During transcription, RNA polymerase generates positive super helical tension in the DNA ahead of it and negative super helical tension behind it due to its movement along the DNA.

• A is incorrect: DNA supercoiling does regulate gene expression, but it also plays an important role in genome organization, so it is not accurate to say it does not affect genome organization.

• D is incorrect: Human topoisomerase I makes a transient break in only one strand of the DNA, not both, to relieve supercoiling, so this statement is wrong.

• E is incorrect: Topoisomerase II cuts both strands of the DNA, not just a single strand. It allows for relaxation by making a break in both strands, so this statement is also incorrect.

Information Booster:

1. Circular DNA in mitochondria, bacteria, and viruses is negatively supercoiled to stabilize the structure and help in DNA processes.

2. RNA polymerase generates supercoiling in DNA: positive supercoiling ahead and negative supercoiling behind, which is essential for transcription.

3. Topoisomerase I relieves supercoiling by breaking only one strand of the DNA.

4. Topoisomerase II relieves supercoiling by making a break in both strands of DNA.

5. DNA supercoiling impacts gene expression and genome organization, both of which are crucial for maintaining cellular functions.

Additional Information:

• Statement A: DNA supercoiling regulates both gene expression and genome organization, so stating it does not impact genome organization is incorrect.

• Statement D: Topoisomerase I makes a break in only one strand of the DNA, while topoisomerase II cuts both strands to relieve supercoiling, making statement D incorrect.

• Statement E: The mechanism described for topoisomerase II is inaccurate because it cuts both strands, not just a single strand.