In circular DNA topology, the number of times the two strands are intertwined is called the linking number. What is this quantity?

Introduction / Context:
DNA topology in covalently closed circular DNA is described by geometric quantities that remain conserved unless strands are broken. Recognizing these terms is essential to understanding supercoiling and enzymatic action by topoisomerases.

Given Data / Assumptions:

• DNA is circular and covalently closed.
• We need the term that counts how many times the two strands wind around each other.

Concept / Approach:
The linking number (Lk) is an integer describing total interlinking of the two strands. It is decomposed as Lk = Tw + Wr, where Tw is twist (local helical turns) and Wr is writhe (supercoiling). Lk changes only when a strand is cut and rejoined.

Step-by-Step Solution:
1) Identify quantity tied to interlinking of both strands: linking number.2) Distinguish from twist (twisting number), which measures helical turns along the axis.3) Note writhe (not listed) measures supercoiling of the axis in space.4) Therefore, the correct term is linking number.

Verification / Alternative check:
In relaxed B-DNA of N base pairs, Lk ≈ N / 10.5 when writhe is zero; experimental manipulation with topoisomerases demonstrates Lk changes only via strand passage.

Why Other Options Are Wrong:
Helix frequency: vague and not a standard conserved topological invariant.Twisting number: refers to Tw component, not total interlinking.None of these: incorrect because “linking number” is the accepted term.

Common Pitfalls:
Equating twist with linking number, ignoring writhe, or assuming Lk can change without strand breakage.

Final Answer:
Linking number