Final answer:
The distance from the phosphorus atom in the DNA backbone to the center of the helix is 1.0 nm, which is the radius of the DNA helix with a total diameter of 2.0 nm.
Step-by-step explanation:
According to the double helix model of DNA, developed by James Watson and Francis Crick, the distance from the phosphorus atom of the backbone to the center of the axis is 1.0 nanometer (nm). This measurement refers to the radius of the DNA helix, which corresponds to half of the total diameter.
Watson and Crick used X-ray crystallography data, which revealed that the DNA helix has a width of 2.0 nm. Therefore, since the sugar-phosphate backbones are on the outside of the double helix, forming the structural framework, the radius from the phosphorus atom to the helical axis would be half the width of the whole DNA molecule.