Topoisomerases are enzymes that solve a fundamental physical problem: DNA gets twisted, tangled, and knotted during normal cell activity, and something has to undo that. Every time your cells copy DNA or read a gene, the double helix winds tighter ahead of the machinery doing the work. Topoisomerases relieve that tension by cutting DNA strands, allowing the twist to release, and then resealing the break. Without them, DNA would become so overwound that replication and gene expression would grind to a halt.
Why DNA Gets Tangled in the First Place
DNA is a double helix, meaning two strands are coiled around each other. When a protein moves along DNA to copy it or read a gene, it has to pry the two strands apart. But because the strands are intertwined, separating them at one point forces the DNA ahead to coil even tighter, like twisting a rubber band that’s already wound up. This excess winding is called positive supercoiling, and it creates real physical resistance. Left unchecked, the DNA ahead of the replication or transcription machinery becomes so tightly wound that the process stalls.
Behind the moving machinery, the opposite can happen: the DNA becomes underwound, creating negative supercoils. And when an entire chromosome is copied, the two new DNA molecules can end up physically linked together like chain links. All of these problems require topoisomerases to fix.
How Topoisomerases Cut and Reseal DNA
The basic strategy is deceptively simple: cut the DNA, let it unwind or pass through itself, then seal the break. But the enzyme has to do this without letting the broken DNA fall apart or get damaged, so it stays covalently attached to the cut end the entire time, acting as a temporary molecular clamp.
There are two main families. Type I topoisomerases cut just one strand of the double helix. They don’t need any energy input because the torque stored in the overwound DNA powers the unwinding itself. The enzyme essentially holds onto the broken strand while the intact strand rotates through the gap, releasing tension. Friction between the protein and DNA controls how fast this happens, preventing the DNA from spinning wildly. Single-molecule experiments have measured E. coli topoisomerase I relaxing roughly one supercoil per second under moderate tension, with each catalytic cycle taking about 4 to 15 seconds depending on conditions.
Type II topoisomerases are more dramatic. They cut both strands of the double helix simultaneously, creating a temporary gate. A second segment of DNA is then passed through that gate before the break is resealed. This requires ATP, making it an energy-consuming process. The ability to pass one DNA segment through another is what makes type II topoisomerases uniquely capable of untangling two separate DNA molecules that have become interlinked.
Keeping DNA Replication on Track
During DNA replication, a protein complex called the replisome barrels along the chromosome, unzipping the two strands so each can be copied. Positive supercoiling builds up ahead of this advancing fork. Both type I and type II topoisomerases work ahead of the fork to remove this overwinding and keep the path clear. Most models place topoisomerase I in front of the fork and topoisomerase II behind it, though topoisomerase II also shows a strong preference for relaxing positive supercoils and can work ahead of the fork as well.
Behind the fork, a different problem emerges. As replication nears completion, the two newly copied DNA molecules can become catenated, meaning they’re physically interlocked. Only type II topoisomerases can resolve this, because untangling two separate double-stranded molecules requires cutting through both strands of one to let the other pass through.
Enabling Gene Expression
The same overwinding problem occurs during transcription, when RNA polymerase moves along a gene to produce an RNA copy. Positive supercoils accumulate ahead of the polymerase and negative supercoils build behind it. Topoisomerases on both sides keep the DNA at a manageable tension level. In yeast, researchers have found that cells maintain sufficient topoisomerase levels to ensure neighboring genes can be expressed properly. When topoisomerase activity drops, the supercoiling from one gene’s transcription can physically inhibit the gene next door from being read.
Separating Chromosomes Before Cell Division
One of topoisomerase II’s most critical jobs comes just before a cell divides. After a cell copies its entire genome, the two identical sets of chromosomes (called sister chromatids) are tangled together. Topoisomerase IIα unlinks these entangled chromatids in a process called decatenation, which is essential for the chromosomes to pull apart cleanly during mitosis.
Cells take this step so seriously that they have a built-in checkpoint. If the chromatids aren’t sufficiently untangled, the cell actively delays division. Research in human fibroblasts has shown that depleting topoisomerase IIα severely reduces the cell’s ability to decatenate DNA, delays the transition into mitosis, and when cells do eventually divide without proper decatenation, the result is abnormal chromosome segregation and genomic instability, a hallmark of cancer.
Topoisomerase as a Drug Target
Because topoisomerases are essential for cell survival and because they work by temporarily breaking DNA, they present an elegant target for both cancer drugs and antibiotics. The strategy is the same in both cases: trap the enzyme in its DNA-cutting state so the temporary break becomes permanent.
Cancer Chemotherapy
Several widely used chemotherapy drugs work by poisoning human topoisomerases. Topoisomerase I inhibitors like irinotecan and topotecan (FDA-approved for colon, lung, and ovarian cancers) prevent the enzyme from resealing its single-strand cut. As these breaks accumulate, they block DNA replication and trigger cell death. Topoisomerase II inhibitors, including the anthracyclines doxorubicin and daunorubicin, intercalate into DNA and stabilize the double-strand break that the enzyme creates. The resulting DNA damage activates the cell’s self-destruct program. Because cancer cells divide rapidly and depend heavily on topoisomerase activity, they’re especially vulnerable to these drugs.
Antibiotics
Bacteria have their own type II topoisomerases: DNA gyrase and topoisomerase IV. Fluoroquinolone antibiotics, one of the most prescribed antibiotic classes worldwide, target both of these enzymes. They bind at the interface between the enzyme and DNA, blocking the resealing of double-strand breaks. The trapped enzyme-DNA complex then acts as a physical barrier to the replication fork and transcription machinery, and the stabilized breaks can become permanent, killing the bacterial cell. Fluoroquinolones tend to hit gyrase harder in gram-negative bacteria and topoisomerase IV harder in gram-positive bacteria, though this varies by drug. Because human topoisomerases are structurally different from bacterial ones, these antibiotics can selectively kill bacteria without poisoning the patient’s own enzymes.
Type I vs. Type II at a Glance
- Type I: Cuts one DNA strand. No ATP required. Relaxes overwinding and underwinding. Works by letting the cut strand rotate around the intact one. Subdivided into types IA, IB, and IC based on structure and mechanism.
- Type II: Cuts both DNA strands. Requires ATP. Can untangle two separate DNA molecules by passing one through the other. Essential for separating chromosomes before cell division. Subdivided into types IIA and IIB.
Both types are found across all domains of life, from bacteria to humans, reflecting how fundamental the problem of DNA topology is. Any organism that needs to replicate, transcribe, or segregate DNA needs topoisomerases to manage the physical consequences of working with a twisted, intertwined molecule.