Name: how topoisomerases untwist dna
Uploaded: 2020-11-23T11:47:35.000Z
Duration: 2 min 2 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Biology

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

9-12.B.1.3

DNA Topoisomerases

The unwinding of the DNA double helix during replication results in overwinding in regions ahead of the replication fork.&#160;
Additionally, supercoiling can occur when the DNA twists back on itself due to the inability of the DNA ends to freely rotate to relieve the torsional stress. This twisting inhibits further unwinding of the DNA, stalling vital cell processes such as DNA replication.&#160;
To deal with this winding problem, cells have enzymes known as topoisomerases that have both nuclease and ligase activity; that is, these enzymes reversibly break then reconnect phosphodiester bonds in DNA to remove torsional strain in overwound DNA.
Topoisomerases fall into two categories.&#160;
Type I topoisomerases are ATP-independent and act by cutting a bond between nucleotides on a single strand of double stranded DNA.&#160;
The unbroken DNA strand is then passed through the gap in the broken strand into the upper cavity of the enzyme.&#160;
Finally, the enzyme ligates the broken ends of the DNA strands back together and produces a locally relaxed DNA molecule.
Type II topoisomerases, on the other hand, are ATP-dependent and act on supercoiled DNA where the DNA is tangled around itself.&#160;
The enzyme creates a double strand break in one loop of the DNA double helix before helping the unbroken loop to pass through this break via an ATP-dependent reaction.
Then, using the energy from a second ATP, Type II topoisomerase reseals the broken ends of the DNA strands and finally detaches from the DNA- leaving behind an untangled DNA helix.

dna topoisomerases

Topoisomerases are enzymes that relax overwound DNA molecules during various cell processes, including DNA replication and transcription. These enzymes regulate positive and negative DNA supercoiling without changing the nucleotide sequence. DNA overwinding in a clockwise direction results in positively supercoiled DNA, whereas underwinding in a counterclockwise direction produces negatively supercoiled DNA.
Types and Mechanism of action
Topoisomerases are divided into two main types.&#160; Type I Topoisomerases act on one strand of double-stranded DNA, and they are further divided into three categories: IA, IB, and IC.&#160; Type IA forms a covalent bond with 5&#39; end of the cleaved DNA strand and removes negative supercoils. It creates a single-strand breakthrough which the opposite strand can pass, resulting in a locally untangled DNA molecule. Type IB forms a covalent bond with the 3&#39; end of the broken DNA strand and can relax both positively and negatively supercoiled DNA. The enzyme rotates the cut single-strand around the opposite strand, untwisting the DNA in the process.&#160; Type IC is mainly found in archaea and has a similar mechanism as type IB.&#160; Most Type I topoisomerases do not require ATP to relax supercoiled DNA.&#160;With the exception of reverse gyrase, this unique type IA topoisomerase does require ATP and generates positive DNA supercoils rather than unwinding them.
Type II topoisomerases are ATP-dependent enzymes that cut both strands of a DNA double helix. They are divided into two subtypes, Type IIA and Type IIB. These subtypes are distinguished by their structure and locations of their protein domains. Both subtypes have similar mechanisms; they transfer a loop of intact supercoiled DNA through the broken double-strand, thereby untangling the coil by one loop. Bacterial gyrase, which belongs to the Type IIA topoisomerases, can introduce negative supercoiling in the DNA and thus is different from all other known topoisomerases.&#160;&#160;

Topoisomerases are enzymes that relax overwound DNA molecules during various cell processes, including DNA replication and transcription. These enzymes ...

Education

JoVE Core

Molecular Biology

Unit 1

DNA Replication

KEY TERMS AND CONCEPTS

how topoisomerases untwist dna

DNA topoisomerases are enzymes that untwist DNA when it becomes overcoiled during cell processes such as DNA replication and transcription. They help regulate positive and negative DNA supercoiling without changing the nucleotide sequence. Clockwise overwinding creates positively supercoiled DNA, while counterclockwise underwinding produces negatively supercoiled DNA.

Topoisomerases are grouped into two main types. Type I topoisomerases act on one strand of double-stranded DNA, and they include IA, IB, and IC. Type IA forms a covalent bond with the 5' end of the cut DNA strand and removes negative supercoils. It makes a single-strand break that lets the other strand pass through, which leaves the DNA locally untangled.

Type IB forms a covalent bond with the 3' end of the broken strand. It can relax both positively and negatively supercoiled DNA. The enzyme rotates the cut strand around the opposite strand to untwist the DNA. Type IC is found mainly in archaea and works in a similar way to type IB.

Most type I topoisomerases do not need ATP to relax supercoiled DNA. ATP is a molecule that provides energy for many cell reactions. One exception is reverse gyrase, a special type IA topoisomerase that does require ATP and creates positive DNA supercoils instead of removing them.

Type II topoisomerases are ATP-dependent enzymes that cut both strands of the DNA double helix. They are divided into type IIA and type IIB, which differ in protein structure and in the location of their protein domains. Both subtypes work by passing a loop of intact supercoiled DNA through the broken double strand, which untangles the coil one loop at a time. Bacterial gyrase is a type IIA topoisomerase that can introduce negative supercoiling, making it different from other known topoisomerases.

How Topoisomerases Untwist DNA

replication in prokaryotes

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replication in eukaryotes

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Okazaki Fragments in DNA Replication

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DNA Replication Teams in the Replisome

DNA replication depends on the replisome, a coordinated complex of proteins that copies DNA with high accuracy. This DNA replication machinery brings many parts together so the leading and lagging strands can be made at the same time. A key idea in this process is the trombone model, proposed by Bruce Alberts in 1980. In this model, a loop forms on the lagging strand when a primer is made. The loop grows as an Okazaki fragment is synthesized, then falls apart when that fragment is finished.

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Mismatch repair helps fix DNA copying errors after replication. When DNA is copied, small mistakes can slip through. This repair system finds those mismatches and corrects them before they become permanent changes. The process depends on a group of mutator proteins. These proteins work together to detect the error, remove the incorrect section, and help replace it with the proper DNA sequence. In this way, mismatch repair supports the accuracy of genetic information. If the repair system does...

DNA topoisomerases are enzymes that untwist DNA when it becomes overcoiled during cell processes such as DNA replication and transcription. They help regulate positive and negative DNA supercoiling without changing the nucleotide sequence. Clockwise overwinding creates positively supercoiled DNA, while counterclockwise underwinding produces negatively supercoiled DNA. Topoisomerases are grouped into two main types. Type I topoisomerases act on one strand of double-stranded DNA, and they...

telomeres and telomerase

How Telomerase Protects Chromosome Ends

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Animal Mitochondrial DNA and Inheritance

Animal mitochondrial DNA has its own genome, separate from nuclear DNA. It is a double-stranded, closed circular molecule with about 20,000 base pairs. One strand is the heavy, or H-strand, because it is rich in guanine. The other is the light, or L-strand, because it is rich in cytosine. Compared with nuclear DNA, mitochondrial DNA has very little non-coding sequence. It also has no introns. Its genes are packed closely together, and some even overlap. The D-loop is the main non-coding...

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Eukaryotic cells can carry three genetic systems: nuclear, mitochondrial, and chloroplast. Over time, many genes from mitochondria and chloroplasts have moved into the nucleus. In some plant species, this gene transfer is still happening. This movement has affected large parts of the genome in some organisms. About 18% of the Arabidopsis thaliana nuclear genome is thought to come from the chloroplast's cyanobacterial ancestor. In yeast, around 75% of the genome is derived from the...

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Single-strand DNA-binding proteins, or SSB proteins, help protect DNA during replication. When double-stranded DNA is unwound, the two strands separate. SSB proteins bind to the exposed single strands and keep them stable. These proteins bind in a sequence-independent manner. That means the order of the nitrogenous bases does not have to follow a specific pattern for SSB proteins to attach. Their binding straightens single-stranded DNA and makes it more rigid. This support may help DNA...

key terms and concepts

Learn how DNA topoisomerases relieve overcoiling during replication and transcription, and compare type I and type II enzymes, including gyrase.

How Topoisomerases Untwist DNA | JoVE K12

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How Topoisomerases Untwist DNA

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