What Is Topoisomerase in DNA Replication?

Topoisomerases are enzymes that untangle and relieve tension in DNA during replication. Every time a cell copies its DNA, the two strands of the double helix must be pulled apart, and this creates a mechanical problem: the DNA ahead of the copying machinery winds up tighter and tighter, like twisting a rubber band until it buckles. Without topoisomerases stepping in to cut, swivel, or pass strands through one another, replication would grind to a halt within seconds. These enzymes are found in every domain of life and have become major targets for both antibiotics and cancer drugs.

The Twisting Problem That Topoisomerases Solve

DNA is a double helix, meaning its two strands wrap around each other in a spiral. During replication, a protein complex called a helicase pries the strands apart at a structure called the replication fork. But here is the catch: as the helicase unwinds the DNA at the fork, the DNA further ahead becomes overwound. Think of holding a twisted rope at two points and pulling the middle apart. The regions you are not holding twist even more tightly. In molecular terms, this creates what researchers call positive superhelical stress ahead of the advancing fork.

That buildup of torsional strain is not a minor inconvenience. If left unresolved, the overwound DNA ahead of the fork compacts so severely that the replication machinery stalls. The advance of a replication fork requires unwinding of the parental double helix, generating positive superhelical stress that must be relaxed by topoisomerases for replication to continue.¹ If the torsion is still not dealt with, it can even force the DNA into unusual structures at the fork itself, further blocking progress.²

Topoisomerases evolved specifically to handle this. They temporarily break one or both strands of the DNA, allow the tension to dissipate, and then seal the break. The result is that the double helix relaxes enough for the replication fork to keep moving. Different types of topoisomerases accomplish this in distinct ways, which matters for understanding how drugs exploit them.

Type I Topoisomerases Cut One Strand

Type I topoisomerases work by nicking just one of the two DNA strands. Once the nick is made, the intact strand can rotate through or around the break, releasing torsional strain. The enzyme then reseals the nick. No energy input from ATP is needed; the stored energy in the overwound DNA drives the relaxation on its own.

Within this broad category, there are subfamilies that work quite differently from each other at the structural level. Type IA enzymes, found in bacteria and also in human cells, pass the intact strand through the break. Type IB enzymes, which are the primary relaxation engines during replication in human cells, use a mechanism researchers describe as “controlled rotation.” One DNA strand swivels around the other while the enzyme holds on, and friction between the protein and the DNA slows the swivel enough that the relaxation happens in discrete steps rather than all at once.³ There is even a third subfamily, type IC, found so far only in certain extreme-environment microorganisms. These three subfamilies are not closely related to one another evolutionarily, despite all cutting a single strand.⁴

Getting topoisomerase I to the right place at the right time is not automatic. In human cells, a chromatin-remodeling protein called BAZ1B is needed to load topoisomerase I onto the replication fork. When researchers depleted BAZ1B from cells, topoisomerase I recruitment to replication forks was abolished, even though the enzyme still worked fine elsewhere, such as during gene transcription.⁵ This hints at how tightly regulated the whole process is: the cell does not just flood the nucleus with topoisomerases and hope for the best.

Type II Topoisomerases Cut Both Strands

Type II topoisomerases take a more dramatic approach. They make a temporary double-strand break in one segment of DNA, then pass a second intact DNA double helix through the gap before resealing it. This requires energy from ATP hydrolysis.⁶ The ability to pass one double helix through another makes these enzymes uniquely suited for solving a problem that type I enzymes cannot easily handle: the tangling of newly copied chromosomes.

When a circular bacterial chromosome is replicated, the two daughter molecules end up linked together like chain links, forming structures called precatenanes. Without a type II topoisomerase to unlink them, the cell cannot pull the two new chromosomes apart before dividing. In the bacterium E. coli, topoisomerase IV performs this job, trailing behind the replication fork and disentangling the freshly copied DNA as it goes.⁷ Without this enzyme, chromosome segregation fails and cells die.

Bacteria also have another type II enzyme, DNA gyrase, which is unusual because it can actively introduce negative supercoils into DNA, not just relax positive ones. This is important for keeping the bacterial chromosome compacted and for facilitating the initial opening of the helix at the origin of replication. Human cells have two type IIA topoisomerases (topoisomerase IIα and IIβ) that handle similar unlinking duties but lack the supercoiling ability of gyrase.

How Antibiotics Exploit Bacterial Topoisomerases

The fact that bacterial topoisomerases differ from human ones makes them excellent drug targets. Fluoroquinolones, one of the most widely prescribed antibiotic classes (including drugs like ciprofloxacin and levofloxacin), work by targeting DNA gyrase and topoisomerase IV. These drugs do not simply block the enzymes from working. Instead, they trap the enzymes in a complex with cleaved DNA, preventing the break from being resealed. The result is permanent double-strand breaks scattered across the bacterial chromosome, which is lethal.⁸

When fluoroquinolones bind to gyrase or topoisomerase IV in the presence of DNA, they alter the enzyme’s shape, boosting DNA cleavage while reducing religation. The enzymes become trapped in three-way complexes with the drug and broken DNA.⁹ This mechanism, converting an essential enzyme into a weapon against the cell’s own genome, is remarkably effective. It also explains why fluoroquinolones are bactericidal (they kill bacteria) rather than merely bacteriostatic (slowing growth).

Topoisomerase-Targeting Cancer Drugs

The same “turn the enzyme against its own DNA” strategy is used in cancer chemotherapy, except the targets are human topoisomerases. Two major classes of anticancer drugs exploit this vulnerability.

Drugs targeting topoisomerase I, such as the camptothecin derivatives irinotecan and topotecan, trap the enzyme on cleaved DNA in the same way that fluoroquinolones trap bacterial enzymes. Camptothecin itself was discovered decades ago, and its derivatives remain powerful anticancer agents because of their ability to stabilize topoisomerase I-DNA cleavage complexes. However, they have clinical limitations, partly due to the chemical instability of a key structural feature needed for the trapping interaction.¹⁰

Drugs targeting topoisomerase II include widely used agents like doxorubicin and etoposide. Researchers draw an important distinction between topoisomerase II “poisons” and “catalytic inhibitors.” Poisons increase the levels of enzyme-DNA cleavage complexes, effectively converting the enzyme into a DNA-cutting machine that cannot finish its job. Catalytic inhibitors, by contrast, simply shut down the enzyme’s activity without causing extra breaks. Every clinically relevant topoisomerase II drug identified to date acts as a poison, not a catalytic inhibitor.¹¹ The poisoning approach is more effective at killing cancer cells, though it also accounts for some of the toxicity these drugs cause in healthy tissue.

Why Cancer Cells Develop Resistance

A persistent challenge with topoisomerase-targeting chemotherapy is that cancer cells often develop resistance over time. Several mechanisms contribute. Cells can ramp up efflux pumps (ABC transporters) that actively expel the drug before it reaches its target. They can also acquire mutations in the topoisomerase genes themselves, altering the enzyme’s shape so the drug no longer traps it effectively. Other resistance strategies include reducing how much drug accumulates inside the cell, inactivating the drug chemically, and activating survival pathways like NF-κB signaling.¹² Drugs like doxorubicin and topotecan remain clinically useful against many tumor types, but resistance frequently limits long-term success.¹³

Repairing the Damage When Topoisomerases Get Stuck

Even without drugs involved, topoisomerases occasionally fail to complete their cut-and-reseal cycle. When a topoisomerase I gets stuck on DNA with a broken strand, the enzyme remains covalently attached, forming what is called a DNA-protein crosslink. This is dangerous for the cell: the stuck enzyme blocks replication and can trigger mutations or cell death if not removed.

Cells have dedicated repair enzymes to handle exactly this situation. Tyrosyl-DNA phosphodiesterases (TDP1 and TDP2) can clip the bond holding the stuck topoisomerase to the DNA, freeing the strand for normal repair.¹⁴ Interestingly, TDP2, which was originally thought to specialize in cleaning up topoisomerase II-linked damage, can also step in to help repair topoisomerase I crosslinks when TDP1 is absent, providing a backup system.¹⁵ This redundancy makes sense from an evolutionary standpoint: topoisomerase accidents are common enough that cells need more than one way to deal with them.

Topoisomerases Beyond the Replication Fork

Replication gets most of the attention, but topoisomerases also manage DNA topology during transcription (when genes are read to make RNA) and during recombination (when DNA strands are exchanged between chromosomes). During transcription, RNA polymerase generates the same sort of torsional stress that the replication fork does. Topoisomerases relax this stress and also help prevent the formation of R-loops, three-stranded structures where newly made RNA invades the DNA double helix. Human cells encode six topoisomerase enzymes, and several of them play roles in suppressing or resolving R-loops to maintain genome stability.¹⁶

In recombination, a specific type IA topoisomerase plays a starring role within a multi-protein machine called the dissolvasome. When homologous recombination produces tangled four-way DNA junctions (called double Holliday junctions), the dissolvasome uses a helicase to migrate the junctions toward each other and a type IA topoisomerase to unlink the final intertwined strands, cleanly separating the two DNA molecules without any crossover.¹⁷ The topoisomerase in this complex acts as a decatenase, passing single strands through one another to achieve unlinking.¹⁸

Topoisomerases in Mitochondrial DNA

Your mitochondria, the energy-producing compartments inside your cells, carry their own small circular genomes. These genomes also need to be replicated and transcribed, which means they face the same topological tangles as nuclear DNA, just in a much smaller space. Mitochondria use a dedicated topoisomerase I (called Top1mt) whose activity concentrates at the origins of mitochondrial DNA replication, suggesting a direct role in initiating and managing the replication process.¹⁹

Mitochondria also rely on a type IA topoisomerase called Top3α to separate freshly replicated mitochondrial DNA molecules. When Top3α is dysfunctional, mitochondrial DNA molecules aggregate together and the total copy number drops, which has been observed in patients with genetic deficiencies in this enzyme. Additionally, the type II enzyme Top2β regulates mitochondrial DNA replication and transcription by altering DNA topology, a detail worth noting because topoisomerase II inhibitors used in cancer chemotherapy could inadvertently damage mitochondrial DNA as a side effect.²⁰

Viruses Carry Their Own Topoisomerases

Some viruses, particularly poxviruses, encode their own topoisomerases rather than relying entirely on the host cell’s enzymes. Poxviruses are unusual among DNA viruses because they replicate in the cytoplasm of the infected cell, far from the nucleus where host topoisomerases reside. Their virus-encoded type IB topoisomerase helps release the torsional strain generated during replication and transcription of the viral genome.²¹

Surprisingly, this viral topoisomerase is not absolutely essential for replication in cell culture. When researchers knocked out the topoisomerase gene in vaccinia virus, the mutant virus could still replicate, but it formed fewer and smaller plaques, had reduced early gene transcription, and showed delayed DNA replication. The primary role of the enzyme appears to be boosting early transcription, which happens inside the virus core shortly after infection.²² Because the topoisomerase acts so early in the infection cycle, drugs that trap it on nicked DNA could theoretically serve as antiviral agents if they can penetrate the virus core. Researchers have already identified at least one natural compound, sansalvamide A from marine organisms, that inhibits a poxvirus topoisomerase’s DNA relaxation activity in laboratory assays.²³

The Surprising Evolutionary Origins of Topoisomerases

One of the more fascinating aspects of topoisomerase biology is how many times nature independently invented these enzymes. The five main families of topoisomerases (IA, IB, IC, IIA, and IIB) are not all descended from a common ancestor. They arose independently, and some share protein building blocks that were apparently recruited separately into different enzyme designs.²⁴ This independent origin makes sense when you consider that any organism with double-stranded DNA, whether a bacterium, an archaeon, or a eukaryote, faces the same physics of helical winding and needs some tool to manage it.

Where eukaryotes got their primary type IIA topoisomerase is a question that has recently gotten a surprising answer. Phylogenetic analysis suggests that eukaryotic type IIA topoisomerase was not inherited from the ancestral cell lineage at all but was instead acquired from ancient giant viruses related to modern Nucleocytoviricota, before the last common ancestor of all eukaryotes even existed. Early proto-eukaryotes apparently used a different enzyme, a type IIB topoisomerase, to manage their DNA topology and only later swapped in the viral-origin enzyme that all modern eukaryotes now rely on.²⁵ The idea that a core piece of the human DNA replication toolkit traces its ancestry to a virus is a striking reminder of how intertwined viral and cellular evolution really are.

Watching Single Molecules in Real Time

Much of what we know about how topoisomerases work comes from experiments on purified proteins and DNA in test tubes. But in recent years, single-molecule techniques have added a level of detail that bulk experiments cannot provide. Magnetic tweezers, for instance, let researchers apply precisely calibrated forces and twists to individual DNA molecules while watching a single topoisomerase enzyme do its job. These experiments have confirmed that type IB topoisomerases relax supercoils in discrete rotational steps and have revealed how friction between the enzyme and DNA governs the speed of relaxation.²⁶ Being able to watch one enzyme molecule at a time, rather than averaging across billions, has helped resolve longstanding questions about the mechanism that structural studies alone could not settle.