7.1 DNA Structure and Replication

7.1 Making careful observations—Rosalind Franklin’s X-ray diffraction provided crucial evidence that DNA is a double helix. (AHL)

Photo 51 is Rosalind Franklin’s famous image of the DNA double helix. When he saw this image, James Watson is reported to have said, ” The instant I saw the picture my jaw fell open and my pulse began to race.”  To the trained eye, this cryptic, black-and-white image immediately reveals the helical structure of DNA, as well as the relative positions of the sugar-phosphate backbone and the nitrogen bases. It was the scientific evidence needed to understand the structure of DNA.

Watson and Crick, although they knew that this meant a helix, were not fluent in the chemical knowledge that would allow them to calculate ratios and distances.  This information was provided by Maurice Wilkins, their co-recipient of the Nobel Prize and Rosalind Franklin’s co-worker. Additionally, it was Franklin’s work as part of a chemical report in 1952 that finally convinced them to put the bases on the inside, rather than the outside. While Wilkins shared in the Nobel Prize, Rosalind Franklin did not. Her research was not published until after Crick and Watson’s article and so she was not cited in their list of references (though she was thanked in the acknowledgements section). Speculation remains that she was taken advantage of to one degree or another and  did not receive the accolades she deserved.  The controversy was only heightened as she died at age 37 in 1958 and so was ineligible for the Nobel Prize.


“Secret of Photo 51.” PBS. PBS, 2003. Web. 17 Oct. 2014.  <http://www.pbs.org/wgbh/nova/photo51/&gt;.

All original research papers from 1953 on the discovery of the structure can be downloaded as PDFs from the journal Nature.


One thought on “7.1 DNA Structure and Replication

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s