Catenated DNA Enforces Sister Chromatid Cohesion

Every time a human cell divides to create two new cells, it must perform a feat of extreme precision: it must split its genetic material, the DNA, into two perfectly identical halves. This process, known as mitosis, is the engine of growth and repair in the body. If the machinery fails, the resulting cells might receive too many or too few chromosomes, a condition called aneuploidy. This imbalance is a hallmark of cancer and can lead to developmental disorders. To prevent this chaos, the cell relies on a molecular glue called the cohesin complex. This ring-shaped protein holds the two newly copied strands of DNA, called sister chromatids, tightly together from the moment they are made until the very last moment of division. For decades, scientists believed that this protein glue was the sole force holding the sisters together, and that the moment the cell was ready to divide, an enzyme would simply cut the glue, allowing the sisters to be pulled apart by the cell's internal skeleton.

However, a new study from researchers at the University of Copenhagen challenges this long-held view. They discovered that the protein glue is not enough on its own to keep the sisters together against the powerful pulling forces of the cell. Instead, the two strands of DNA are also physically tangled with one another, like links in a chain. These tangles, known as catenanes, act as a secondary, physical lock that works alongside the protein glue. The researchers found that even if the protein glue is cut, the sisters will not separate if these DNA tangles remain intact. It is only when the cell's machinery untangles these DNA links that the sisters can finally part ways. This finding suggests that the cell uses a dual-lock system to ensure that genetic material is never split prematurely, a discovery that reshapes our understanding of how cells maintain their genetic stability.

The story of this discovery begins with a puzzle that has lingered in cell biology for some time. When scientists look at cells under a microscope just before they divide, they often see the two sister chromatids lying very close together, almost touching, in a configuration that looks like a railroad track. Even though the protein glue holding them together has been cut in some experimental conditions, the sisters do not fly apart as expected. They remain stubbornly linked. This observation led the researchers to hypothesize that something else must be holding them together. They suspected that the DNA strands themselves were intertwined. Imagine two long, flexible ropes that have been twisted around each other many times; even if you cut the tape holding them side-by-side, the twists themselves will keep them connected until you actively untwist them. In the cell, these twists are the catenanes, and the researchers set out to prove that they are essential for keeping the sisters together.

To test this idea, the team engineered human cells to carry a special version of the cohesin protein. This version contained a specific weak point that could be cut by a custom-made enzyme called TEV protease. The researchers grew these cells and then introduced the enzyme to cut the cohesin glue while the cells were held in the middle of division. In cells where the DNA tangles were allowed to be resolved naturally, the sisters separated immediately after the glue was cut, confirming that the enzyme worked. However, the researchers then took a different approach. They isolated the chromosomes from these cells and cut the glue in a test tube, but without the cell's natural machinery to untangle the DNA. Even after the glue was completely destroyed, the sister chromatids remained stuck together. They only separated when the researchers added a mechanical force, like vigorous shaking, or when they added an enzyme that could cut the DNA itself. This experiment provided the first direct evidence that the protein glue alone is insufficient to hold the sisters together against physical stress; the DNA tangles are doing the heavy lifting.

The researchers then looked closer at these tangles inside living cells. They identified structures they called sister centromere linkages, which are the physical connections between the two halves of the chromosome at their central point. Using high-resolution imaging, they saw that these linkages were coated with a specific protein called PICH, which is known to bind to stretched DNA. They also found that an enzyme called Topoisomerase II, which is responsible for untangling DNA, was present at these sites. Crucially, they observed that these linkages only disappeared when the cell's internal skeleton, the spindle, pulled on the chromosomes with force. When the researchers blocked the activity of the untangling enzyme using a drug called ICRF-193, the sisters refused to separate, even after the protein glue was cut. The drug essentially locked the DNA tangles in place, preventing the cell from dividing. In contrast, a different drug that breaks the DNA temporarily but allows it to be reconnected did not stop the separation. This proved that the physical integrity of the DNA tangles, not just the presence of the enzyme, was the key factor.

The study also explored what happens when the cell is forced to wait too long in the middle of division. Normally, if a cell is stuck in this phase, the protein glue eventually wears out, a phenomenon known as cohesion fatigue, and the sisters separate prematurely. The researchers found that when they blocked the untangling enzyme, the cells could stay in this waiting phase for much longer without the sisters falling apart. The DNA tangles acted as a backup system, holding the sisters together long after the protein glue had failed. Conversely, when they used a drug that weakened the DNA tangles, the sisters separated much faster than usual. This confirmed that the tangles are not just a passive byproduct of DNA replication but an active, force-resistant component of the cell's division machinery.

Perhaps the most striking evidence came from an experiment where the researchers used a molecular tool called CRISPR to cut the DNA specifically at the sites where these tangles were known to form. They targeted a specific sequence of DNA found in the center of the chromosomes. When they cut this DNA in cells where the protein glue had already been removed, the sisters separated immediately, even when the untangling enzyme was blocked. This showed that the physical connection of the DNA itself was the final barrier holding the sisters together. Once that barrier was broken, the sisters could no longer resist the pulling forces of the cell.

The implications of this work are significant for our understanding of how cells function. It suggests that the cell has evolved a redundant, two-part security system to ensure that chromosomes do not split too early. The protein glue provides a regulated, chemical lock that can be turned off at the right time, while the DNA tangles provide a physical lock that resists the immense forces generated during division. The cell only releases this second lock when it is ready, using the pulling force of the spindle to guide the untangling enzyme to do its job. If this system fails, the result can be the uneven distribution of chromosomes, which is a primary driver of cancer. By revealing that DNA catenation is a critical, active player in this process, the researchers have corrected a long-standing assumption in the field. They have shown that the separation of sister chromatids is not merely a matter of cutting a protein ring, but a complex mechanical event where the physical state of the DNA itself plays a decisive role. This new model helps explain why some cells fail to divide correctly and offers a fresh perspective on the fundamental mechanics of life.