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Nucleic acids were discovered in the mid-1800s.
Phoebus Levene found the parts of DNA: sugar, phosphate, and base — he called it a nucleotide.
But he thought DNA was too simple to carry genetic information. -
Frederick Griffith worked with two types of bacteria: Smooth (S): dangerous → mice died Rough (R): harmless → mice lived When he mixed dead S bacteria with live R bacteria, the mouse still died.
Something in the S bacteria changed the R bacteria.
This was called transformation. -
They used enzymes to destroy either proteins or DNA. When protein was destroyed → transformation still happened. When DNA was destroyed → transformation did not happen. Conclusion: DNA causes transformation. DNA carries genetic information. -
Erwin Chargaff studied DNA from different organisms.
He found that: Amount of A = amount of T Amount of C = amount of G This became known as Chargaff’s rules.
But he didn’t realize how important this was. -
Watson and Crick made a model of DNA.
But their first idea was wrong — they put the parts in the wrong place.
Other scientists like Linus Pauling also made incorrect models. -
They used viruses (bacteriophages) to test if DNA or protein was the genetic material. Viruses with radioactive protein → protein stayed outside the bacteria. Viruses with radioactive DNA → DNA went inside the bacteria. Conclusion: DNA, not protein, is the genetic material. -
Rosalind Franklin used X-ray images to study DNA.
She took Photo 51, which showed a clear double helix shape.
Maurice Wilkins shared this photo with Watson and Crick without her permission. -
Using Photo 51, Watson and Crick corrected their model.
They published the double helix structure of DNA in 1953.
Three papers were published in Nature: Watson Crick Wilkins Franklin Gosling Rosalind Franklin died in 1958 and did not receive a Nobel Prize. -
DNA is a double helix with: A backbone of sugar and phosphate Base pairs in the middle: A with T, and C with G Two strands going in opposite directions (anti-parallel) The sequence of bases gives the instructions to build proteins. -
The discovery of DNA involved many people and many steps.
Watson and Crick often get the credit, but others like Griffith, Avery, Chargaff, Franklin, and more were very important.
DNA is an amazing molecule — the key to life.