Learn more about topoisomerase, an enzyme that is an invaluable tool for DNA replication.

Discover why the spiraling shape of DNA lends to the need for topoisomerase.

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DNA Structure

DNA codes for the genetic information of organisms. Shaped like a double helix, the basic structure of DNA winds itself much like a spiral staircase. In bacteria, this double-stranded spiral forms a circular shape. In larger organisms, DNA lies inside a nucleus and is bundled together in chromosomes. Chromosomes generally form an X shape, and this X is made up of tightly-wound, double-stranded, spiraling DNA.


In order for a cell to divide, it must also replicate its DNA.

As DNA replicates, a new strand is added next to one of the two existing DNA strands. But this process could potentially end up with tangled DNA, much like a tangled telephone cord. Special machinery is required to replicate DNA, since it is wound upon itself, or supercoiled. To remedy the burden of replicating this tangled mess, an enzyme called topoisomerase is employed.

Topoisomerase is like a DNA surgeon which can both cut, or nick, DNA and repair the breakage. Topoisomerase cuts DNA at a particular point and unravels the twist in order to relieve the supercoil. This allows the physical space for another strand of DNA to be synthesized, free from worry of tangling.A new strand of DNA is built upon one of the existing DNA strands at the nicked site, known as the replication fork. If there are two replication forks occurring at the same time, then new DNA strands can be built in two directions. It’s more typical for both bacteria and larger organisms to copy DNA using bidirectional replication (think of ‘bi’, meaning ‘two’) than unidirectional replication (‘uni’, meaning ‘one’).

Topoisomerase I

There are two types of DNA surgeons; they are called topoisomerase I and topoisomerase II.

Each of these enzymes has their own specialty. Topoisomerase I is able to cut a single-stranded gap in one side of the existing DNA supercoil. This makes room for a new strand of DNA to pass across the existing DNA template. Much like a surgeon, topoisomerase I is able to stitch up the DNA, or ligate, when the process is complete.

Topoisomerase II

Topoisomerase II has the tricky job of untangling the coiled DNA and nicking both strands of DNA using its two sets of ‘jaws.’ These strong jaws cleave, or split, not just one strand of DNA as in the case of topoisomerase I, but both strands at once. The resulting hole made in the DNA structure allows space for another double-stranded DNA nearby in the supercoil to slide though.

Similar to a back door, the second strand is not cut, but pushed out when the enzyme opens its second set of ‘jaws.’ This process unweaves the DNA. When the replication process is complete, topoisomerase ligates the cleaved DNA.

Lesson Summary

Depending on the size of the organism, DNA can be formed into a circle or an X-shaped chromosome tightly wound into a supercoil. Since the existing DNA functions as a template for new DNA synthesis, this shape needs to be remedied before replication can occur. Topoisomerase is a valuable enzyme for untangling supercoils and making space for new DNA strands to be created. Topoisomerase can both cleave DNA at a desired replication site and also ligate the DNA once the process is complete.

Topoisomerase I is capable of nicking a single strand of DNA, while topoisomerase II is able to nick the double-stranded structure in its entirety.


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