Question

What happens to DNA (circular DNA or linear DNA that is not free to rotate) when it becomes overwound?

a. It becomes negatively supercoiled.
b. It breaks into a number of fragments.
c. It becomes positively supercoiled.
d. It stretches.
e. Its mass decreases

Answer 1

When DNA becomes overwound, it leads to the formation of positively supercoiled DNA, which is a tighter coil than in its relaxed state.

Step-by-step explanation:

When circular DNA or linear DNA that cannot freely rotate becomes overwound, it becomes positively supercoiled. This refers to the DNA being wound more tightly than its relaxed state, which is typically one turn of the helix per 10 base pairs. Such overwinding puts additional tension on the DNA molecule. Enzymes like topoisomerases can relieve this tension by cutting the DNA, allowing it to unwind, and then rejoining the cut ends. In the case of eukaryotes, DNA is wrapped around histones and further organized into chromatin structures, which can also undergo supercoiling.