Which DNA form is the common right-handed helix?

• A. A-DNA Is The Common Right-Handed Helix
• B. Z-DNA Is The Common Right-Handed Helix
• C. B-DNA Is The Common Right-Handed Helix
• D. RNA-DNA Hybrids Are The Common Right-Handed Helix

Answer: (C)

The main idea is that DNA can fold into different helical forms with distinct shapes and directions, and only one is the standard form in living cells. The canonical right-handed form under physiological conditions is B-DNA. It is the typical structure found in human and other cells, featuring about 10 base pairs per turn and a diameter around 2 nanometers, with a major groove and a minor groove that enzymes and proteins recognize. A-DNA is also right-handed but appears mainly in dehydrated conditions or in RNA-DNA hybrids; it has about 11 base pairs per turn and a broader, shorter profile, so it’s not the usual form in cells. Z-DNA is left-handed and forms only under particular sequences and conditions, not as the common structure. RNA-DNA hybrids tend to adopt an A-form geometry rather than B-form. So the form that stands as the standard right-handed helix in typical biological settings is B-DNA.