Cell division is one of life's most fundamental processes. Before a cell can split into two, it must make an exact copy of its DNA—the instruction manual of life. This is no small feat. The DNA double helix has to unwind and partially deconstruct itself, with the help of many enzymes and molecules, so that each strand can be duplicated. Errors in this copying process can lead to diseases like cancer. A precise picture of all the molecules and steps involved in DNA replication and repair is still the subject of ongoing research.

A recent study from the Indian Association for the Cultivation of Science (IACS), Kolkata, found that during cell division, the enzyme Tyrosyl-DNA phosphodiesterase 1 (TDP1)—usually in charge of clearing certain DNA knots- is temporarily "fired from its job." In its place, the cell brings in MUS81, a DNA processing enzyme that solves the problem in a different way.

"Normally, the job of untangling DNA before duplication is handled by an enzyme called Topoisomerase 1 (Top1)," says lead author Srijita Paul Chowdhuri. "It works like a zipper pull—making tiny cuts to release twists and knots, then sealing the DNA back up." Sometimes, however, Top1 gets stuck mid-job, forming what scientists call Top1 cleavage complexes (Top1ccs).

These complexes block essential processes like DNA replication and transcription and need to be removed. Else, DNA copying, chromosome breakage, and long-term genome instability may result, possibly leading to cancer.

TDP1 acts like a rescue crew, cutting away the trapped Top1 so that DNA replication can continue. For most of the cell cycle, this system works well. But when Paul Chowdhuri and team looked during mitosis—the stage when chromosomes are pulled apart—they noticed TDP1 was missing from the chromosomes entirely.

To find out why, they searched for chemical changes on TDP1 specific to mitosis. Using mass spectrometry and mutational analysis, they discovered that the cell cycle regulator cyclin-dependent kinase 1 (CDK1) adds a phosphate 'tag' on TDP1 at a site called serine 61. This tag acts as an eviction notice, forcing TDP1 to leave the chromosomes.

The researchers then engineered two mutant versions of TDP1. One version, called S61A, could not receive the phosphate tag, so it never got the signal to leave and kept working on the chromosomes during mitosis. The other, called S61D, mimicked a permanently tagged state, behaving like an employee who has already packed up and left. In mitotic cells, the S61A version stayed stuck on chromosomes, while the S61D version stayed away. This proved the phosphate tag was the signal to remove TDP1 from chromosomes.

The team also tested what happens if TDP1 refuses to leave. Cells with the S61A version had more DNA breaks, chromosome bridges, and fragments during and after mitosis. "Unlike TDP1, MUS81 doesn't directly cut Top1 off the DNA. Instead, MUS81 steps in when the DNA copying machinery gets stuck. It cuts at these stalled sites, which releases the built-up stress on the DNA and can even help the machinery restart in the right direction. This action has also been linked to a process called Mitotic DNA Synthesis (MiDAS)—a last-minute way for cells to finish DNA replication and repair just before they divide. That connection made us curious to test whether MUS81 was also involved in fixing Top1 problems specifically during mitosis," says Paul Chowdhuri.

When the researchers reduced MUS81 levels in these cells, the damage decreased—showing that the problem was a clash between TDP1 and MUS81.

During most of the cell cycle, TDP1 is essential for clearing Top1 traps. But during mitosis, it is removed so that MUS81 can take over through the fast, specialised repair process of MiDAS. This switch helps avoid repair conflicts and keeps the cell's genetic material intact.

But why does TDP1 have to step aside and let MUS81 take charge during mitosis? "TDP1 can remove Top1 from DNA, but it needs help from other proteins to finish the full repair process. During mitosis, however, chromosomes are usually less responsive to DNA repair, and many of these helper proteins may not be active. That could be why TDP1's role is limited at this stage. Once mitosis is over, it's possible that another, yet-to-be-identified enzyme removes the phosphate tag from TDP1, allowing it to return to its normal function," says Paul Chowdhuri.

This study shows that even repair systems need good timing—sometimes it is about knowing when to step aside so the right tool can take over. Studying the handover between TDP1 and MUS81 adds a new piece to how cells keep DNA safe during division. In the future, understanding these job switches could help develop ways to target cancer cells while sparing healthy ones and inspire the search for similar handoffs in other repair systems.