Question

Define supercoiling, positive supercoil, topoisomerase, and negative supercoil.

Short answer

Supercoiling is the winding of DNA strands. Positive supercoiling involves over-winding. Topoisomerase manages supercoiling by cutting DNA. Negative supercoiling involves under-winding.

Step-by-step solution

- Define Supercoiling

Supercoiling refers to the over- or under-winding of a DNA strand, and is an expression of the strain on that strand.

- Define Positive Supercoil

Positive supercoiling occurs when the DNA helix is over-wound. This generally happens when a DNA molecule has more turns in its helix than a relaxed DNA molecule.

- Define Topoisomerase

Topoisomerase is an enzyme that helps manage DNA supercoiling by cutting the DNA strand to add or remove turns, thereby relieving tension.

- Define Negative Supercoil

Negative supercoiling occurs when the DNA helix is under-wound. This means there are fewer turns in the DNA helix compared to a relaxed DNA molecule.

Key concepts

Supercoiling

Supercoiling is a concept that describes the over-winding or under-winding of a DNA strand. Imagine a piece of rubber band twisted in your hands. If you twist it too much, it coils up on itself. This additional coiling or looping is what we call supercoiling in DNA. Supercoiling helps the DNA to fit within the small confines of a cell and impacts how genes are expressed and replicated.
Supercoiling is categorized based on whether the DNA is over-wound or under-wound. This structural change helps in regulating DNA metabolism.

Positive Supercoil

Positive supercoiling occurs when the DNA helix is wound too tightly, accumulating more turns than the DNA naturally has. To understand positive supercoiling, think about twisting a phone cord until it becomes too tight and starts looping over itself. Positive supercoiling can occur during DNA replication and transcription, as the DNA in front of the replication or transcription machinery tends to become over-wound. This can hinder the machinery’s progression if not managed properly.

Topoisomerase

Topoisomerase is an essential enzyme that helps manage DNA supercoiling by cutting and rejoining the DNA strands. Here’s how it works:
• When DNA becomes too supercoiled (either positively or negatively), topoisomerase steps in.
• It makes a temporary cut in one or both strands of the DNA.
• By doing so, it allows the DNA to unwind or rewind to a more relaxed state.
• Finally, it reseals the DNA breaks.
Without topoisomerase, the supercoiling could cause knots and tangles that interfere with critical processes like replication and transcription. There are different types of topoisomerases, each with specific roles in managing DNA structure.

Negative Supercoil

Negative supercoiling occurs when the DNA helix is under-wound, meaning there are fewer turns in the DNA compared to its relaxed state. Think of it as stretching a coiled spring, where the coils are pulled apart. Negative supercoiling is quite common in living cells because it makes the DNA easier to separate into single strands, which is necessary for replication and transcription.
Cells often maintain a negatively supercoiled state to facilitate these processes. Negative supercoiling helps reduce the energy required to unwind DNA for access by various enzymes and proteins involved in gene expression.