Question

List three mechanisms that relax the twisting stress in helical DNA molecules.

Short answer

Topoisomerases, DNA gyrase, and helicases relax the twisting stress in helical DNA molecules.

Step-by-step solution

Understanding DNA Supercoiling

DNA supercoiling refers to the over-twisting or under-twisting of the DNA helix, which generates twisting stress in the molecule.

Role of Topoisomerases

Topoisomerases are enzymes that help to manage DNA supercoiling. They cut one or both strands of the DNA, allow it to unwind, and then reseal the break.

Function of DNA Gyrase

DNA gyrase is a type of topoisomerase found in bacteria that introduces negative supercoils into DNA, counteracting positive supercoiling and thereby relaxing the twisting stress.

Involvement of Helicases

Helicases are enzymes that unwind DNA strands. By breaking the hydrogen bonds between base pairs, they help reduce the twisting stress during processes like DNA replication and transcription.

Key concepts

Topoisomerases

Topoisomerases are essential enzymes that play a critical role in managing DNA supercoiling, which is the over-twisting or under-twisting of the DNA double helix. These enzymes work by cutting one or both strands of the DNA molecule, allowing it to unwind and release the built-up stress.
There are two main types of topoisomerases:

• Type I Topoisomerases: These enzymes cut one strand of the DNA, allowing the other strand to pass through the break, which helps reduce supercoiling.
• Type II Topoisomerases: These enzymes cut both strands of the DNA and pass another segment of the helix through the break, which helps manage more severe supercoiling.

Once the DNA has been relaxed, the topoisomerases reseal the breaks, ensuring that the DNA remains intact. This process is crucial for various cellular activities such as replication, transcription, and chromosome segregation.

DNA gyrase

DNA gyrase is a specific type of Type II topoisomerase that is primarily found in bacteria. It plays a unique role in introducing negative supercoils into DNA, which helps counteract the positive supercoiling that occurs during processes like DNA replication and transcription.
Negative supercoiling is important because it makes the DNA molecule more compact and easier to manage within the confines of the cell. Here are a few key points about DNA gyrase:

• Mechanism: DNA gyrase works by making double-strand cuts in the DNA and then passing another part of the helix through the break before resealing the cuts. This introduces negative supercoils.
• Importance in Antibiotics: Many antibiotics target DNA gyrase, inhibiting its function and thereby preventing bacterial cell division and proliferation.

By successfully managing DNA supercoiling, DNA gyrase ensures the smooth progression of critical cellular processes and contributes to the overall stability and functionality of bacterial genomes.

Helicases

Helicases are vital enzymes that unwind DNA strands by breaking the hydrogen bonds between the nucleotide base pairs. This unwinding is crucial for processes like DNA replication and transcription because it allows access to the single-stranded DNA template.
Here’s how helicases function and why they are important:

• Mechanism: Helicases use energy from ATP hydrolysis to move along the DNA and separate the two strands, creating a replication fork where other proteins can interact with the single-stranded DNA.
• Role in DNA Replication: During DNA replication, helicases open up the DNA helix so that DNA polymerases can copy each strand, thus ensuring the genetic information is accurately passed on to new cells.
• Role in Transcription: In transcription, helicases unwind the DNA to allow RNA polymerase to read the DNA template and synthesize RNA.

By unwinding the DNA, helicases help to alleviate the twisting stress associated with supercoiling, facilitating smooth progression of essential cellular processes.