Which type(s) of DNA have a right-handed helix?

• A. B-form and A-form
• B. A-form only
• C. B-form only
• D. Z-form

Answer: (A)

A right-handed DNA helix twists clockwise as you move along the strand. The two common right-handed forms are B-DNA and A-DNA. B-DNA is the standard form in physiological conditions and is the familiar textbook structure. A-DNA is also right-handed and occurs in dehydrated conditions or in RNA-DNA hybrids; it’s more compact and the base pairs are tilted relative to the helix axis. Z-DNA, on the other hand, is left-handed, twisting counterclockwise under certain sequences and supercoiling conditions. So the right-handed forms include both B-form and A-form, while Z-form is left-handed.