Which chemical group did Franklin place near the outside of DNA?

Which chemical group did Franklin place near the outside of DNA? Phosphate groups.

While analysing DNA’s X-ray diffraction patterns, Rosalind Franklin concluded that the phosphate groups belonged near the outside of the molecule. This placed the negatively charged sugar-phosphate backbone on the exterior, with the nitrogen-containing bases oriented toward the interior.

The conclusion was scientifically important because some earlier ideas had placed the phosphates inside the structure. An exterior phosphate backbone also helped explain DNA’s interaction with water and positively charged ions. The bases could then pair internally, a geometry that ultimately supported the double-helix model.

Franklin recorded evidence for a helical structure and an external phosphate arrangement in her research notes. Her work did not by itself supply every detail of the final model, but it sharply constrained the possible structures. James Watson and Francis Crick’s 1953 model incorporated the outside-backbone arrangement, together with complementary base pairing.

The common mix-up is to confuse phosphate groups with the bases. Adenine, thymine, guanine and cytosine form the paired interior, whereas phosphate groups alternate with sugars to make the external backbone. Hydroxyl, methyl and amino groups are not the defining outer framework of DNA.