Twist

Definition

In biophysical chemistry, twist refers to the amount of helical winding of a DNA or RNA strand around its axis. This concept is crucial in understanding how nucleic acids maintain their structural integrity and function, as the twist influences their supercoiling and overall topological properties. The degree of twist affects how tightly or loosely the strands are coiled, which in turn impacts processes such as replication and transcription.

5 Must Know Facts For Your Next Test

1. The twist in a double helix typically corresponds to 10.5 base pairs per turn, which is considered the standard helical structure for B-form DNA.
2. Changes in the twist can lead to supercoiling, which affects the overall energy state and biological functions of nucleic acids.
3. Enzymes like topoisomerases are essential for managing twist and supercoiling during processes like DNA replication and transcription.
4. Twist is often described in terms of positive or negative values, indicating whether the strands are overwound or underwound, respectively.
5. The balance between twist, supercoiling, and linking number is essential for maintaining proper cellular function and genetic information flow.

Review Questions

• How does the concept of twist influence the supercoiling of DNA?
  • Twist directly affects supercoiling because it determines how tightly or loosely the DNA strands are coiled around each other. When the twist increases, it can lead to overwinding, causing positive supercoiling, while a decrease in twist can result in underwinding and negative supercoiling. This relationship is crucial for understanding how DNA maintains its structure during processes like replication and transcription.
• What role do enzymes like topoisomerases play in managing the twist of nucleic acids?
  • Topoisomerases are enzymes that help manage the twisting and coiling of DNA by introducing or removing twists. They can cut one or both strands of DNA to relieve torsional stress caused by overwinding or underwinding. By controlling the twist, topoisomerases ensure that the proper level of supercoiling is maintained, which is essential for efficient replication and transcription.
• Evaluate how changes in twist and supercoiling might affect cellular processes such as gene expression.
  • Changes in twist and supercoiling can significantly impact gene expression by altering the accessibility of DNA to transcription factors and RNA polymerase. If DNA becomes too tightly supercoiled due to increased twist, it may be less accessible for transcription, leading to decreased gene expression. Conversely, relaxed DNA with appropriate twisting may enhance accessibility, facilitating efficient transcription and ultimately influencing cellular responses and functions.

Related terms

Supercoiling: A phenomenon that occurs when the DNA double helix is overwound or underwound, resulting in additional twists that can affect the DNA's functional state.

Topological Constraints: Refers to the limitations placed on the structure of nucleic acids due to their physical arrangement and spatial characteristics, which influence interactions with proteins.

Linking Number: The total number of times one strand of DNA winds around the other, a key value that determines the topology of a DNA molecule.