“Because Photograph 51 is an important clue to the chemical nature of DNA. So, moving on, in 1929, Phoebus Levene, an American biochemist, showed that in addition to Kossel’s nucleobases, nuclein had a sugar molecule and a phosphate group. He named this combination of a nucleobase, a sugar molecule, and phosphate, a ‘nucleotide’. But, Levene didn’t stop here. He went on to propose that each DNA molecule had a tetranucleotide structure, or consisted of one set of A, T, G and C. Clear?” questioned Grandpa.
Three scientists confirmed that Griffith’s ‘transforming molecule’ was DNA
(a) Oswald Avery.
Credits: Owned by Rockefeller Archive Center
(b) Colin MacLeod.
Credits: Owned by National Institute of Health
(c) Maclyn McCarty (to the extreme right).
Credits: Owned by Acc. 90-105 - Science Service, Records, 1920s-1970s, Smithsonian Institution Archives, and uploaded by Giac83 on Wikimedia
“Clear”.
By 1934, two Swiss scientists, Torbjorn Caspersson and Einar Hammersten, showed that DNA was a polymeric molecule with long stretches of nucleotides that were not necessarily in multiples of four. So, of course, the tetra-nucleotide structure was rejected. Then, in 1937, William Astbury took the first photograph of DNA through X-ray crystallography, using a sample supplied by Caspersson.”
Mittu sighed. “Is photograph 51 next?”
Hershey and Chase’s transduction.
T2 phage is a virus that infects the bacteria, Escherichia coli (E.coli). To confirm that DNA
was the transforming molecule in Griffith’s experiment, Hershey and Chase labelled the DNA
of a virus (T2 phage) with radioactive phosphorus and its protein capsid with radioactive
sulphur. When the radioactively labeled virus was used to infect E.coli, Hershey and Chase
found phosphorous labelled DNA in the infected cells. This confirmed that it was DNA, and not
protein, that was the ‘molecule of life’.
Credits: Graham Beards, Wikimedia Commons.
A nucleotide has a nucleobase (upper right), a sugar (centre) and a phosphate group (left).
Credits: Cacycle, Wikimedia Commons.
Phoebus Levene identified the chemical structure of nucleotides.
Credits: Uploaded by Materialscientist, Wikimedia Commons.
Grandpa ignored the interruption. “In 1950, Erwin Chargaff discovered that the cells of all organisms showed a 1:1 ratio of purine (adenine and guanine) and pyramidine (thymine and cytosine) bases. He also proved that A always pairs with T, and G always pairs with C.”
“Why A-T and not A-C?”
“Stable hydrogen bonds can only be formed between A-T and G-C, but not otherwise. In 1951, Edward Ronin suggested that DNA may have phosphate groups at its centre and nucleobases jutting outwards. Then, came a discovery in 1952.”
“Too many details. Can we just skip all this?” Mittu snapped.
“Sure,” said Grandpa innocently.
“Photograph 51 was taken in 1952. But let us skip that part.”
Mittu growled.
Grandpa chuckled. “This photograph was taken by Rosalind Franklin using x-rays.”
“A woman scientist in 1952?”
“Yes. A very accomplished yet underrecognized scientist.”
“Under-recognized?”
“Yes. If you check your textbook, it’s very likely that you’ll find a copy of photograph 51, but without any mention of the fact that Franklin was responsible for taking it.”
“But, why?”
“Because there were many scientists racing towards identifying the structure of DNA. Franklin was only one among them. In fact, it’s likely that she had nearly cracked the structure of DNA with this Photograph. Looking at it, she had identified DNA to be a double helix — that’s your mysterious X,” said Grandpa.
“This is a photo of a DNA molecule?!” Mittu looked at the image again. Then, “what is a double helix, Grandpa?”

“Well, every molecule of DNA is composed of two strands of nucleotides that are anti-parallel to each other. Franklin was aware of Chargaff’s rule. Based on the ‘X’ in this photo, she had, correctly, deduced that the nucleobases were on the inside of the helix and the phosphate groups were on the outside (unlike earlier models). And, nucleobases on these strands were held together in pairs through hydrogen bonds. She is also supposed to have deduced that each base pair is 3.4 angstrom apart.”
“Did she write about this?” asked Mittu.
Rosalind Franklin used x-rays to photograph DNA.
Credits: National Portrait Gallery, London.
“She intended to, but wanted to wait till she had more evidence. In the meanwhile, she showed this photograph to a colleague, Maurice Wilkins. Wilkins was very keen on deducing the structure of DNA. So keen that his complaints had convinced Sir Braggs, head of the Molecular Biology Laboratory at Cambridge, to ask two scientists who worked there — James Watson and Francis Crick — to abandon their pursuit of the structure of DNA. Wilkins also turned down Linus Pauling’s request to see Photograph 51. Pauling, a Nobel Laureate in protein chemistry, needed this evidence to confirm the ‘triple helical’ structure for DNA that he had proposed. Since Wilkins had declined his request, Pauling based his model on Astbury’s images instead. The race got harder. Sir Braggs advised Watson and Crick to rejoin it. On a visit to King’s college, Watson coaxed Wilkins into showing him Photograph 51. Wilkins did this without Franklin’s knowledge or consent. As soon as he saw it, Watson recognized the significance of this photograph. Combining what they knew from the photograph and Pauling’s protein structure, Watson and Crick built the first correct model of DNA in 1953. Watson, Crick and Wilkins were jointly awarded the Nobel Prize for Medicine in 1962 for this achievement.”

The double helix.
Credits: Forluvoft, Wikimedia Commons.
License: CC-0.
“And, Rosalind Franklin?”
“She remained the ‘Dark Lady of DNA’.”
Mittu looked at the photograph and sighed, “That is so unfair!”
Both remained silent for a while. Then, Mittu asked, “What is the point of all this discovery Grandpa?”
“Well, it helps us understand DNA replication, protein synthesis from DNA, DNAs role in disease, its use in treating diseases and also its application in solving crimes.”
Mittu looked up at Grandpa questioningly.
“Francis Crick proposed the ‘central dogma’ in 1957. Central dogma explains the flow of information from DNA to Proteins through RNA. According to it, DNA first passes on information to an intermediate molecule, RNA. RNA is then translated into the amino acids that make up proteins. Crick also proposed that the information for each amino acid was encoded in the sequence of three consecutive bases on each strand of DNA. This is called the ‘genetic code’.”
Comments
Dear Dr. Rohini Chintha , I
Dear Dr. Rohini Chintha , I have gone through your article on the discovery of DNA structure. You have beautifully portrayed the histrory of DNA structure. All the discoveries that lead to the elucidation of the DNA structure has been nicely presented in your article chronologically. In the paragraph you wrote about Phoebus Levene, I think I would be better if you could mention about the discovery of Ribose sugar and deoxyribose sugar by him in the year 1909 and 1929 respectively. Above all the article has been written in a nice way that will surely help students to get an idea about the structure of DNA, the secret of life.