Question

One of the following nucleic acids has a left handed helix:

• A. m-RNA
• B. t-RNA
• C. A-DNA
• D. Z-DNA

Hint:

Z-DNA is a left-handed helix, while most naturally occurring DNA exists in the right-handed B-DNA form.

Answer 1

The Correct Option is D

m-RNA does not form an extended double helix on its own, and t-RNA's helical stem regions, like A-DNA, adopt the standard right-handed helical geometry.

Z-DNA is unique among these in having a zig-zag sugar-phosphate backbone that twists in the left-handed direction, which is also where it gets its name.

So the nucleic acid conformation with a left-handed helix is Z-DNA.

Answer 2

Another angle is to think about the biological conditions under which each of these forms.

1. m-RNA: Formed by transcription as a flexible, largely single-stranded molecule that folds locally rather than adopting one uniform helical handedness throughout its length.
2. t-RNA: Its helical arms form under standard physiological conditions and behave like short right-handed A-form RNA duplexes, the same handedness seen in typical double-stranded RNA regions.
3. A-DNA: Forms under dehydrating conditions but remains right-handed, just more compact than B-DNA.
4. Z-DNA: Forms particularly in alternating purine-pyrimidine sequences under conditions such as high salt concentration, and specifically adopts a left-handed helical winding rather than the right-handed winding seen in every other option here.

Only Z-DNA is associated with the distinctive left-handed helical winding among these nucleic acid structures.

The correct answer is Z-DNA.