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

Step 1: Understanding the different forms of DNA.
DNA can exist in several forms, including A-DNA, B-DNA, and Z-DNA. Z-DNA is a left-handed helix, unlike the more common B-DNA which is right-handed.
Step 2: Analyzing the options.
(1) m-RNA: mRNA is a single-stranded RNA molecule and does not exhibit a left-handed helix structure.
(2) t-RNA: tRNA also has a distinct three-dimensional structure but does not form a left-handed helix.
(3) A-DNA: A-DNA is a right-handed helix.
(4) Z-DNA: Correct — Z-DNA is a left-handed helix, characterized by its zig-zag backbone structure.
Step 3: Conclusion.
The correct answer is (4) Z-DNA, as it is the left-handed form of DNA.

Answer 2

Handedness of a helix depends on the direction its backbone twists, so let's check what's known about each option's structure.

1. m-RNA: Messenger RNA is predominantly single-stranded and does not form an extended helical duplex on its own, so the concept of a left- or right-handed double helix does not really apply to it.
2. t-RNA: Transfer RNA folds into a compact cloverleaf/L-shaped structure stabilized by short double-helical stems, and those stems adopt the standard right-handed A-form helix typical of RNA duplexes, not a left-handed form.
3. A-DNA: A-DNA is one of the right-handed conformations of the DNA double helix, more compact and wider than B-DNA, but still right-handed.
4. Z-DNA: Z-DNA has a zig-zagging sugar-phosphate backbone (where its name comes from) and winds in the left-handed direction, making it the only conformation among these four that is left-handed.

Since m-RNA lacks an extended duplex form, t-RNA's helical stems are right-handed, and A-DNA is right-handed, only Z-DNA fits a left-handed helical structure.

Therefore, the correct answer is Z-DNA.