X-ray diffraction photographs such as this one led to the discovery of the double-helix form of the dna molecule. A model of a small part of a DNA molecule is shown.A human DNA molecule, which is normally coiled and folded into a microscopic package called a chromosome, would be a meter or so long if stretched out. The development and functioning of every living organism is con trolled by the dNa in its cells When the organism reproduces, copies of its DNA are passed on to the new generation