Why A-DNA and B-DNA Are Named A and B: Origins and Context Explained

Why Are A-DNA and B-DNA Called A and B?

A-DNA and B-DNA receive their names from Rosalind Franklin, who arbitrarily assigned the letters “A” and “B” to distinguish between two distinct DNA fiber forms observed under varying humidity conditions. These labels are not based on a scientific classification system but rather a simple alphabetical order.

Origin of the Names

Rosalind Franklin’s X-ray diffraction studies of DNA fibers revealed two distinct structural patterns. She observed that DNA fibers displayed different diffraction images depending on the humidity level surrounding them. At lower humidity, a particular pattern appeared, termed the “A” form, while at higher humidity, a different pattern, called the “B” form, was observed.

The designations “A” and “B” came about as straightforward labels for these two diffraction types. Franklin chose the letters simply because they are the first two in the alphabet, as documented in her communications and early papers.

Primary Research Context

Original reports by Wilkins and Gosling, under Franklin’s supervision, described these two fiber diagrams. They noted:

“Attempts to achieve a further increase in crystallinity by working at still higher relative humidity resulted in the observation that at very high humidities a well-defined structural change occurs, leading to a new type of fibre diagram. The structures represented by these two diagrams will be referred to in what follows as structures A and B.”

This statement confirms the naming was directly linked to the two observed physical states of DNA fibers under different conditions, without any intended deeper meaning behind the choice of letters.

Informal Mnemonics

To aid memory, educators and students sometimes use mnemonics such as:

• A for Anhydrous or dried DNA (non-physiological)
• B for Body form, reflecting DNA in physiological, aqueous environments

These serve as helpful cues but do not represent the historical or scientific origin of the names.

Summary

• A-DNA and B-DNA names arose from Rosalind Franklin’s arbitrary alphabetical labeling of two distinct fiber diffraction patterns.
• The “A” and “B” forms correspond to DNA structures observed at different humidity levels.
• Mnemonic associations exist but are informal and not the basis for the original designations.