The Linking Number (Lk) of a Closed-Circular, Double-Stranded DNA Molecule

Question

The linking number (Lk) of a closed-circular, double-stranded DNA molecule is changed by:
A) breaking a strand, then rejoining it.
B) breaking a strand, unwinding or rewinding the DNA, then rejoining it.
C) breaking all hydrogen bonds in the DNA.
D) supercoiling without the breaking of any phosphodiester bonds.
E) underwinding without the breaking of any phosphodiester bonds.

Accepted Answer

Breaking a strand and unwinding or rewinding the DNA changes the supercoiling of the DNA, which affects the linking number. The linking number is the number of times one strand of DNA crosses over the other in a closed circular molecule, and can be changed by altering the amount of supercoiling. Breaking all hydrogen bonds would separate the two strands and not affect the linking number, and supercoiling or underwinding without breaking any phosphodiester bonds would also not affect the linking number.