Fundamentals of Molecular Biology. Part 2: Molecular Genetic Mechanisms - A. N. Ogurtsov 2011

Key Molecular Genetic Mechanisms
DNA Supercoiling

Many prokaryotic DNAs are circular (cyclic). Circular DNA is also found in eukaryotic Cell/35.html">Mitochondria and METABOLISM/14.html">Chloroplasts.

Each of the two strands of a circular DNA molecule forms a closed loop without free ends.

Local unwinding of the DNA double helix, which occurs during DNA Replication, induces torsional stress in the rest of the molecule because it lacks free ends that could relieve this stress by freely rotating.

As a result, the circular molecule twists into a supercoiled form (Figure 5(a)). In other words, when one part of the DNA is unwound, the other part becomes supercoiled.

The enzyme topoisomerase I, found in Cells, relieves such stresses that arise in circular DNAs during replication or other processes.

Topoisomerase I binds to DNA, cleaves one of the phosphodiester bonds (allowing the supercoiled "twisted" molecule to "relax"), restoring the relaxed circular form (Figure 5(b)), and then "reseals" the broken strand.

Another enzyme, topoisomerase II (also known as DNA gyrase), cleaves both strands of a circular DNA molecule simultaneously and then rejoins them. Consequently, topoisomerase II can either remove supercoils or link two intersecting circular DNA molecules into one at their crossing point.

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Figure 5 - Circular DNAs: a - supercoiled circular covalently closed DNA; b - relaxation of circular DNA upon Cleavage of one strand by topoisomerase I, which subsequently restores strand closure

From a functional standpoint, it is significant that supercoiled DNA possesses a considerable reserve of energy compared to its relaxed form. This energy reserve serves as a "signal" for the binding of certain Proteins (such as topoisomerases).

Although eukaryotic DNA is not circular, the Condensation of DNA into Chromosomes involves loop formation and Chromatin coiling. Therefore, during chromatin decondensation, DNA replication, and Transcription, torsional stress and supercoiling also occur. The presence of topoisomerase I in The eukaryotic Nucleus makes it possible to eliminate such torsional stresses.



Last update: 12/08/2026

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