59. The following feature about A-DNA is INCORRECT
1. The DNA is arranged in a left handed double helix
2. The base pairs in A-DNA are not perfectly perpendicular to the helix axis
3. The DNA is arranged in a right handed double helix
4. Number of base pairs per helical turn is 11

Understanding A-DNA: Structure, Features, and Key Differences from B-DNA and Z-DNA

A-DNA is one of the three main forms of DNA, alongside B-DNA and Z-DNA. It occurs under dehydrating conditions and has unique structural properties that differentiate it from the more common B-DNA. Let’s examine an important question about A-DNA and identify which feature is incorrect.

Answer: Incorrect Feature of A-DNA

Statement: “The DNA is arranged in a left-handed double helix”

✅ Incorrect! A-DNA is actually a right-handed double helix, similar to B-DNA, but with distinct structural differences.

Correct Features of A-DNA:

1. Base pairs in A-DNA are not perfectly perpendicular to the helix axis – ✅ Correct
2. The DNA is arranged in a right-handed double helix – ✅ Correct
3. Number of base pairs per helical turn is 11 – ✅ Correct

Thus, the incorrect feature is option 1 because A-DNA is right-handed, not left-handed.

Structural Features of A-DNA

A-DNA differs from B-DNA in several ways:

1. Helical Handedness

• A-DNA: Right-handed
• B-DNA: Right-handed
• Z-DNA: Left-handed

2. Helical Pitch and Tilt

• A-DNA has a tighter helical turn with 11 base pairs per turn, while B-DNA has 10.5 base pairs per turn.
• The base pairs in A-DNA are tilted and not perfectly perpendicular to the helix axis.

3. Major and Minor Grooves

• A-DNA has a narrower major groove and a wider, shallower minor groove compared to B-DNA.

When Does A-DNA Form?

• A-DNA is typically observed under low humidity or dehydrating conditions.
• It can form in RNA-DNA hybrids and double-stranded RNA.
• Some bacteriophages have been found to store DNA in the A-DNA conformation.

Differences Between A-DNA, B-DNA, and Z-DNA

Feature	A-DNA	B-DNA	Z-DNA
Helical Handedness	Right	Right	Left
Base Pairs per Turn	11	10.5	12
Groove Structure	Narrow Major, Wide Minor	Balanced Grooves	Deep Minor, Flattened Major
Tilt of Base Pairs	Tilted	Perpendicular	Zig-zag conformation

Conclusion

A-DNA is a right-handed double helix with 11 base pairs per turn, occurring under dehydrating conditions. The incorrect statement about A-DNA is that it has a left-handed helix. Understanding these structural differences is crucial in molecular biology, particularly for studies involving DNA-RNA interactions and structural transitions in nucleic acids.