Question

Recall If circular B-DNA is positively supercoiled, will these supercoils be left- or right-handed?

Short answer

Positively supercoiled B-DNA will have right-handed supercoils.

Step-by-step solution

Understand DNA Supercoiling

Supercoiling refers to the over-winding or under-winding of DNA. In B-DNA, if it is positively supercoiled, it means that the DNA is overwound.

Determine the Handedness of Supercoils

Positively supercoiled DNA has extra twists in the same direction as the helix. B-DNA is a right-handed helix, therefore, positive supercoiling will be right-handed.

Conclude the Direction

Since the positive supercoiling maintains the direction of the original right-handed B-DNA, the supercoils will also be right-handed.

Key concepts

B-DNA

B-DNA, or B-form DNA, is the most common form of DNA found in nature. It has a right-handed helix and follows the Watson-Crick base pairing rules. The structure is stabilized by hydrogen bonds between the nucleotides.
The major features of B-DNA include:

• A right-handed double helix
• Approximately 10.5 base pairs per turn
• Wide and deep major groove, narrow and shallow minor groove

B-DNA is the standard reference when scientists discuss DNA structure.

positive supercoiling

Supercoiling refers to the overwinding or underwinding of the DNA helix. Positive supercoiling occurs when the DNA is overwound.
This means that there are extra twists in the same direction as the helices. In the case of B-DNA, which is a right-handed helix, positive supercoiling adds right-handed twists.
Positive supercoiling can create tension in the DNA molecule. This helps during processes like replication and transcription, where the DNA needs to be temporarily unwound.

handedness of DNA

The handedness of DNA refers to the direction in which the DNA helix twists. B-DNA is a right-handed helix. This means as it spirals away from you, it goes clockwise.
Positive supercoiling in B-DNA results in right-handed coiling. This is because additional twists are introduced in the direction of the existing right-handed helix.
Handedness is crucial for the functionality of DNA, impacting how enzymes and proteins interact with the DNA molecule.
Knowing the handedness helps in predicting the behavior and interaction of DNA during various cellular processes.