Illustration of DNA structure

1953 DNA structure

The book of science

Tom Sharp

Francis Crick, James Watson genetics Illustration of DNA structure

DNA structure

Deoxyribonucleic acid is a double helix with antiparallel chains of hydrophilic sugar-phosphate backbones and hydrophobic bases packed into the core. Maurice Wilkins and Raymond Gosling prepared X-ray diffraction images of DNA showing clear “X shaped” patterns. Wilkins and Alex Stokes believed its structure was helical. Rosalind Franklin provided X-ray diffraction images and told Crick and Watson that the backbones had to be on the outside of the structure, but disputed that its structure was helical. Erwin Chargaff discovered that in DNA the number of guanine units equals the number of cytosine units and the number of adenine units equals the number of thymine units and he explained this to Crick and Watson. Crick and Watson recognized the structural simularity of guanine-cytosine and adenine-thymine pairs and predicted that ten nucleotide bases were stacked into the core for each turn of the double helix.

DNA machinery

Helicases split a double helix into strands like opening a zipper. Nucleases cut the resulting strands at specific sites. Polymerases make copies and proofread the results and remove bases that don’t match the DNA template. Ligases rejoin the cut strands.


DNA encodes the instructions for the development, functioning, and reproduction of all known living organisms. Tends to makes me wonder how this complex molecule could be so basic. Multiple strands in a nucleus are long, about 220 million base pairs in the largest human chromosome. However, most of it, 98 percent in human DNA, doesn’t code information for making proteins. In the vast random universe (if the universe were random) a little something has clicked.