Since March is Women in History month, let’s celebrate some women in science. Rosalind Franklin was an English chemist who worked with x-ray crystallography to lay the foundation of Watson and Crick’s knowledge of the structure of DNA.
In the early 1950s, Franklin worked with Maurice Wilkins on x-ray crystallography at King’s College. X-ray crystallography is a method used by chemists to determine the three-dimensional structure of a crystallized molecule. The crystallized molecule is placed in an x-ray tube and is struck with x-rays. X-rays can pass through crystals and interact with the electrons of the atoms in the crystal which causes the x-rays to diffract or scatter. The pattern of scattered x-rays is recorded as dark marks on film and gives clues about the structure of the molecule.
In 1944, Avery, MacLeod, and McCarty isolated DNA and discovered that it contained genes that passed on genetic information, but the structure of DNA and how it worked was difficult to determine. Scientists did know that DNA was composed of nucleotides of deoxyribose, phosphates, and bases of adenine, guanine, thymine or cytosine, but how those nucleotides were arranged was still unknown. Many research scientists around the world were attempting to determine the structure of DNA but none had any success.
Franklin’s most famous photograph was taken using x-ray diffraction of DNA and is known as Photo 51. Franklin was not the first person to use x-ray diffraction on a pure fiber of DNA, but she conducted her experiments differently than previous scientists. Franklin pumped hydrogen gas through a salt solution to keep the DNA fibers surrounded by water which reveals more of the structure of the DNA. Her image was also taken after 62 hours of exposing the DNA to x-rays and revealed far more details than previously achievable.
Franklin’s Photo 51 was able to show that DNA was made of a double helix in which one twist consisted of 10 nucleotides. She also was able to show that the bases AGCT were on the inside of the helix and the phosphate groups were on the outside.
Some controversy ensued as a result of the image. Maurice Wilkins showed Franklin’s DNA image to researchers James Watson and Francis Crick without Franklin’s knowledge (see source 1). Watson and Crick went on to use the information contained in the image to develop their own model of DNA which is now the structure that we understand. Watson and Crick were also able to predict semiconservative replication, later proven by Meselson and Stahl.
Watson, Crick and Wilkins won the Nobel Prize in Physiology or Medicine in 1962 for their discoveries of the structure of DNA. Franklin died in 1958 of ovarian cancer at age 37 and the Nobel Prize is not awarded posthumously.
Image credits: Image of Rosalind Franklin is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.
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