General Science question from JKAS Prelims, 2021 by JKPSC
Which one of the following is not true about the structure of DNA?
Last updated Jun 24, 2026
Correct Answer:
Option B —
DNA is always a right handed helix.
Explanation
While the most common form of DNA (known as B-DNA) is a right-handed helix, it is not "always" right-handed.
Z-DNA: This is a specific form of DNA that forms a left-handed helix. It has a zigzag pattern in its backbone and usually occurs in regions with specific repeating sequences, such as alternating cytosine and guanine.
Biological Role: Z-DNA is thought to play a role in regulating how genes are expressed and in relieving structural tension during certain cellular processes.
Why the other options are true:
A) Double stranded helix with sugar-phosphate backbone: This is the foundational structure of DNA. The "sides" of the ladder are made of alternating sugar and phosphate molecules, while the "rungs" are the nitrogenous bases.
C) Strands run in opposite direction: This is known as being anti-parallel. One strand runs in a 5-prime to 3-prime direction, while the other runs 3-prime to 5-prime. This orientation is essential for DNA replication and base pairing.
D) Major and minor grooves: Because the two strands are not perfectly symmetrical in how they wrap around each other, they create gaps of different sizes. These grooves are critical because they allow proteins (like those involved in copying or reading DNA) to bind to specific sequences.
Answer verified by Quintessence Classes faculty — Karan Nagar, Srinagar.
