21. The Winding Problem (1,069)

As the CMG helicase moves along the double helix, propelling the replisome forward and separating the DNA into leading and lagging strands, it creates a physical problem. DNA cannot simply be pulled apart without consequence. The act of unwinding one region of the double helix necessarily affects the regions ahead of it.

This is often referred to as the “winding problem.”

Let me explain with a human-scale analogy.

Imagine two ropes wound tightly around each other like a double helix. Now fix one end of those ropes firmly to a table. If you step back and begin pulling the ropes apart to unwind them — much as CMG helicase unwinds double-stranded DNA — you will quickly feel resistance. That resistance is torsional stress. And it doesn’t just disappear. Instead, it builds up in the still-wound portion of the rope, creating secondary coils, or “supercoils.”

For my older readers, think of the coiled cord of a phone handset. Twist it repeatedly and you’d see the cord form tight secondary loops. To remove them, you’d let the handset dangle and allow the handset to spin freely, unwinding the cord in the opposite direction.

DNA behaves in much the same way.

As CMG helicase separates the strands, the DNA ahead of the replication fork cannot rotate freely — it is constrained within the chromosome and its surrounding protein environment. So the act of unwinding generates positive supercoils in the duplex DNA in front of the fork.

Roughly every 10–10.5 base pairs that the helicase unwinds introduces one additional helical turn — a +1 change in what is refered to as "linking number" — into the DNA ahead. These extra twists accumulate rapidly. Left unresolved, they would create enormous torsional strain, stall the helicase and thus the replisome, and potentially cause DNA breakage.

So they must be removed.

Type I Topoisomerase (TOP1)

Human cells deploy dedicated molecular machines to solve this problem. These enzymes are called topoisomerases. There are two major types. We begin with Type I topoisomerase, or TOP1 — the primary enzyme that relaxes supercoils ahead of the moving replication fork.

If the problem is overtwisting, the solution — much like the phone cord — is to allow the DNA to untwist. The supercoiled DNA wants to relax. The built-up torsional stress seeks a lower-energy state.

TOP1 makes that relaxation possible.

TOP1 clamps around approximately 20 nucleotides of duplex DNA. Its structure resembles a hinged “C” that opens, encircles the DNA, and then closes securely around it. Once clamped in place, TOP1 performs a tricky maneuver: it cleaves one of the two DNA strands.

This temporary nick allows the cut strand to rotate around the intact strand in a controlled fashion. The rotation is driven not by ATP but by the torsional stress already stored in the DNA itself — much like the spinning phone cord. The intact strand serves as the pivot point; the cleaved strand swivels within the enzyme’s clamp.

Unlike the freely spinning phone cord, though, TOP1 regulates this rotation. It relaxes supercoils incrementally — typically one turn at a time. As the CMG helicase advances and introduces roughly one additional twist every 10 base pairs, TOP1 must continuously remove those twists to maintain the correct topology.

After allowing sufficient controlled rotation, TOP1 re-ligates the nicked strand, restoring the duplex.

TOP1 typically performs multiple rounds of relaxation before dissociating. As the replication fork advances, successive TOP1 molecules bind ahead of it, acting like pressure-release valves that prevent supercoiling from overwhelming the system.

Without TOP1, torsional strain would accumulate rapidly, replication forks would stall, and DNA would be at risk of catastrophic damage.

A final note: TOP1 is an important target in cancer chemotherapy. Drugs like camptothecin, topotecan, and irinotecan stabilize the TOP1–DNA cleavage intermediate, preventing re-ligation. What is normally a transient nick becomes a persistent break. When a replication fork encounters that break, it can convert into a lethal double-strand break. Rapidly dividing cancer cells are particularly vulnerable to this mechanism.

Type II Topoisomerase (TOP2)

There is a second topological problem that arises during replication — this time behind the replication fork.

As leading and lagging strand synthesis proceed, two newly replicated duplex DNA molecules are generated. These duplexes can themselves become intertwined, forming structures known as catenanes — effectively linked rings of double-stranded DNA.

Before cell division, these intertwined chromosomes must be separated. They cannot remain knotted together.

Enter Type II topoisomerase, or TOP2.

TOP2 is larger and more complex than TOP1. Unlike TOP1, which acts on one duplex, TOP2 acts on two double-stranded DNA segments at once.

Its mechanism is elegant and tightly controlled. TOP2 contains three coordinated “gates” — often described as a top (or N) gate, a central DNA gate, and a bottom gate — which open and close in a precise sequence reminiscent of an airlock.

Here is how the process unfolds.

First, one duplex DNA segment enters the central DNA gate. This is the segment that will be temporarily cleaved. The enzyme positions it precisely so that it can cut both strands simultaneously, creating a controlled double-strand break.

Next, a second duplex is captured by closure of the top gate upon ATP binding. ATP binding triggers a conformational change that traps this second duplex securely.

With the second segment held above, TOP2 cleaves the first segment within the central gate, creating a temporary double-strand break. This opening forms a passageway through which the second segment can be transported.

The enzyme then guides the second segment through the break in the first segment and into the lower chamber of the enzyme. Once passage is complete, the first segment is religated, restoring the continuity of that duplex. Only after re-ligation does the bottom gate open, releasing the transported DNA segment.

Finally, ATP hydrolysis resets the enzyme, reopening the top gate and preparing the complex for another cycle.

Each TOP2 molecule can complete roughly one to two strand-passage events per second. Through repeated cycles, intertwined DNA molecules are disentangled, allowing proper chromosome segregation during cell division.

So DNA replication is not merely a matter of copying genetic information. It is also a constant battle against the physical consequences of twisting, pulling, and separating a long helical polymer confined within a cell nucleus.

Topoisomerases are the quiet managers of DNA topology. Without them, the mechanical strain generated by replication would quickly overwhelm the system.

They solve the winding problems — one controlled cut at a time.