What is the Difference Between Positive and Negative Supercoiling of DNA

The key difference between positive and negative supercoiling of DNA is that during positive supercoiling of DNA, the DNA strand is overwound compared to the relaxed state, while during negative supercoiling of DNA, the DNA strand is under wound compared to the relaxed state.

DNA supercoiling is the amount of twist in a DNA strand (over-winding or under winding), and it determines the amount of strain on the strand. Topoisomerases are the enzymes that facilitate and regulate DNA supercoiling to enhance DNA replication and transcription. They are present in all cells, including bacteria and humans. DNA supercoiling is important to many cellular processes such as DNA compacting and gene expression, through regulating access to the genetic code. There are two types of DNA supercoiling as positive supercoiling and negative supercoiling.

CONTENTS

1. Overview and Key Difference
2. What is Positive Supercoiling of DNA
3. What is Negative Supercoiling of DNA
4. Similarities – Positive and Negative Supercoiling of DNA
5. Positive vs Negative Supercoiling of DNA in Tabular Form
6. Summary – Positive vs Negative Supercoiling of DNA

What is Positive Supercoiling of DNA?

Positive supercoiling of DNA is a process where the DNA strand is overwound compared to the relaxed state. This occurs when the double-helical conformation of DNA (right-handed) is twisted tighter (over wound in a right-handed mode) until the helical structure becomes distorted and develops into a level of ‘knot.’

Mathematical equations elaborate on the positive supercoiling of DNA. DNA is not positively supercoiled during normal states but occurs only during cellular processes, for example, to facilitate mitosis where duplicated sister DNAs are segregated into daughter cells during the interphase in the development and maintenance of topologically associating domains. These domains are TADs. Moreover, during mitotic chromosome assembly, it has been shown that condensin induces positive supercoiling of DNA. Condensing is a large protein complex that plays a major role in mitotic chromosome assembly and induces the positive supercoiling in an ATP hydrolysis-dependent manner.

What is Negative Supercoiling of DNA?

Negative supercoiling of DNA is a process where the DNA strand is under wound compared to the relaxed state. This occurs when the double helical conformation of DNA (left-handed) is twisted lighter (under wound in a left-handed mode) until the helical structure becomes relaxed more than the normal relaxed B form of DNA. It facilitates DNA replication and transcription through topoisomerase enzymes.

Supercoiling of DNA produces two types of structures called a plectoneme or a toroid. Sometimes, it could be a combination of both. During negative supercoiling of DNA, the DNA molecule will produce either a two-start right-handed helix with terminal loops (the plectoneme) or a one-start left-handed helix (the toroid). Plectonemes are more common in nature, especially in bacteria.

What are the Similarities Between Positive and Negative Supercoiling of DNA?

• Positive and negative supercoiling of DNA is present in all organisms (from bacteria up to humans)
• Both processes change the shape of the DNA strand.
• They affect the double-helical structure of DNA.
• Moreover, both mechanisms are important for the facilitation of many cellular functions.
• Enzymes control positive and negative supercoiling of DNA.

What is the Difference Between Positive and Negative Supercoiling of DNA?

The key difference between positive and negative supercoiling of DNA is that, during positive supercoiling of DNA, the DNA strand is overwound compared to the relaxed state while during negative supercoiling of DNA, the DNA strand is under wound compared to the relaxed state. Most organisms’ DNA is negatively supercoiled in normal status. Positive supercoiling only occurs during particular cellular functions. Moreover, positive supercoiling of DNA takes place towards the right-hand side, while negative supercoiling of DNA takes place towards the left-hand side.

The below infographic presents the differences between positive and negative supercoiling of DNA in tabular form for side by side comparison.

Summary – Positive vs Negative Supercoiling of DNA

DNA supercoiling is important in many cellular processes, such as DNA compacting and gene expression, through regulating access to the genetic code. It also facilitates and regulates DNA supercoiling to enhance DNA replication and transcription. The key difference between positive and negative supercoiling of DNA is that during positive supercoiling of DNA, the DNA strand is overwound compared to the relaxed state, while during negative supercoiling of DNA, the DNA strand is under wound compared to the relaxed state. Both supercoiling processes of a given strand are elaborated by a mathematical formula. Both processes are compared to a reference state known as the relaxed state of DNA or relaxed B form of DNA.

Reference:

1. Alba, Tyto, et al. “What Is Positive and Negative Supercoiling?” Biology Stack Exchange.
2. Witz, Guillaume, and Andrzej Stasiak. “DNA Supercoiling and Its Role in DNA Decatenation and Unknotting.” Nucleic Acids Research, Oxford University Press.

Image Courtesy:

1. “Subhash nucleoid 06” By Subhash C. Verma, Zhong Qian, Sankar L. Adhya – doi:10.1371/journal.pgen.1008456 Verma SC, Qian Z, Adhya SL (2019) Architecture of the Escherichia coli nucleoid. PLoS Genet 15(12): e1008456. (CC BY 4.0) via Commons Wikimedia
2. “Subhash nucleoid 05” By Subhash C. Verma, Zhong Qian, Sankar L. Adhya – doi:10.1371/journal.pgen.1008456 Verma SC, Qian Z, Adhya SL (2019) Architecture of the Escherichia coli nucleoid. PLoS Genet 15(12): e1008456. (CC BY 4.0) via Commons Wikimedia