The twist, writhe, and linking number distributions in closed circular DNA

Area of Science:

• Biophysical Chemistry
• Molecular Biology
• Structural Biology

Background:

• Circular DNA topology is governed by linking number, twist, and writhe.
• Understanding DNA's structural dynamics is crucial for processes like replication and transcription.
• Single-strand nicks introduce flexibility, altering topological properties.

Purpose of the Study:

• To predict the topoisomer distribution of circular DNA after nick closure, assuming independent twist and writhe.
• To describe the resulting twist and writhe distributions in closed DNA molecules.
• To establish how experimental variances in twist and writhe can be determined.

Main Methods:

• Theoretical modeling of statistically independent twist and writhe distributions.
• Analysis of topoisomer formation upon covalent closure of single-strand nicks.
• Investigation of the dependence of distributions on variance magnitudes.

Main Results:

• The study predicts the nature of topoisomer distributions formed after nick closure.
• It describes the twist and writhe distributions in fully closed circular DNA.
• The relative magnitudes of twist and writhe variances are shown to be experimentally deducible.
• A distinct difference in the temperature coefficient of twist between nicked and closed DNA is predicted.

Conclusions:

• The theoretical framework provides insights into DNA topological transitions.
• Experimental determination of twist and writhe variances is feasible.
• The ratio of twist coefficients is directly linked to the ratio of variances in nicked DNA.
• This work offers a predictive model for DNA structural behavior under topological stress.