Scientists on the path to DNA’s discovery

Friedrich Miescher (1869) He found that the nuclei in pus cells contained a significant amount of material that was not protein. He called it nuclein because it was primarily found in the nucleus. Later renamed DNA after chemical analysis.

Frederick Griffith (1920s) He was trying to develop a vaccine against pneumonia. He took nuclear material from pneumonia infected mice and put it in healthy mice via a viral vector. The healthy mice then got pneumonia and died..

Frederick Griffith This showed the process of transformation, which is the introduction of foreign DNA into a cell by means of a virus or plasmid.

Joachim Hammerling (1930s) Acetabularia are macroscopic algae with 3 distinct regions: foot, stalk and cap. He found if you remove the cap, the cap would grow back, but if you removed the foot, no new foot would grow. He therefore concluded the instructions for making the cap were in the foot (which is where the nucleus was).

Oswald Avery, Maclyn McCarty and Colin McLeod (1944) They improved upon the research done by Griffith and found that it was DNA, not protein, that was responsible for transformation.

Erwin Chargraff (1949) This famous biochemist found that in DNA there were equal amounts of thymine and adenine and equal amounts of cytosine and guanine.

DNA structure was discovered Through the work of those previous scientists, the structure of the nucleotide was discovered… pentose sugar attached to a phosphate group at carbon 5 and a nitrogenous base at carbon 1.

Alfred Hershey and Martha Chase (1952) Performed their famous experiment which used radioisotopes of sulfur and phosphate to show that it is DNA, not protein, that carries the genetic information of the cell.

Rosalind Franklin (1953) Used X-Ray diffraction to take a picture of DNA. This picture showed the double helical structure of DNA. This picture was “borrowed” from her by her co-worker, Maurice Wilkins.

The Photo

James Watson, Francis Crick and the weasel Maurice Wilkins (1953) Wilkins passed the Franklin photo to his drinking buddies Watson and Crick. They were able to explain and prove the double helical structure of DNA.

James Watson, Francis Crick and the weasel Maurice Wilkins (1953) They found that the width of DNA was consistent throughout the molecule at 2 nm. Using Chargraff’s data, they came up with the complimentary base pairing in DNA where a purine always binds to a pyrimidine. A to T and C to G.

James Watson, Francis Crick and the weasel Maurice Wilkins (1953) They proposed that the helix in DNA turned clockwise and was a right handed helix. It would make a complete turn every 10 nucleotides which is a distance of 3.4 nm, so the space between nucleotides is only 0.34 nm.

James Watson, Francis Crick and the weasel Maurice Wilkins (1953) Also they arranged DNA strands to run anti- parallel so that one strand runs in a 5’ to 3’ direction and the other strand runs 3’ to 5’. For all this, they won the Nobel Prize in Franklin who died in 1959, got nothing.