What evidence did Watson and Crick use to determine that DNA is a helix? Is there another cellular macromolecule that can assume a helical shape?
Watson and Crick used a combination of existing evidence and their own model building to determine that DNA is a helix. They were inspired by the X-ray crystallography images of DNA fibers taken by Rosalind Franklin and Maurice Wilkins, which showed a distinctive X-shaped pattern known as the X-ray diffraction pattern.
To interpret the X-ray diffraction pattern, Watson and Crick employed the concept of diffraction by helical structures. They made the important assumption that the structure of DNA consists of two antiparallel helical strands, with the nitrogenous bases positioned inside the helix. They further deduced that the bases must form specific base pairs: adenine (A) with thymine (T) and cytosine (C) with guanine (G). These pairing rules allowed for the complementary base pairing and the double-stranded nature of DNA.
DNA is not the only cellular macromolecule that can assume a helical shape. Another example is RNA (ribonucleic acid), which is closely related to DNA in structure and function. Like DNA, RNA can form helical structures through base pairing, although it is often single-stranded rather than double-stranded.
In addition to DNA and RNA, proteins can also form helical structures. Alpha helices and beta sheets are common examples of protein secondary structure formed by the folding of a polypeptide chain. These helical structures play important roles in protein stability and function.