History of DNA

How it Was Discovered

Friedrich Miescher 1869 (Germany)

• Isolated deoxyribonucleic acid from nucleus of cells– Called this white,

slightly acidic chemical “nuclein”

– Didn’t know it was genetic material

Frederick Griffith 1928 (England)• Working on vaccine for pneumonia bacteria• Discovered that by killing inside of bacteria, it

wouldn’t give disease to mice

Many Scientists 1940

• By this time, many scientists isolated all of the chemicals found in DNA– Phosphate (PO4)

– Sugar (deoxyribose)– Four nitrogen bases (adenine, thymine,

cytosine, guanine)• Now called “deoxyribose nucleic acid”

or DNA

Oswald Avery 1944 (USA)

• Worked with bacteria that causes pneumonia

• Used DNA-destroying enzyme to destroy DNA inside bacteria before inserting it into animal– Animal didn’t get

pneumonia– DNA is genetic material

Erwin Chargaff 1950 (USA)• Biochemist who discovered that ratio

of adenine to thymine was always 1:1 and the ratio of cytosine to guanine was always 1:1 in ALL living organisms

• In human DNA, for example, the four bases are present in these percentages: A=30.9% and T=29.4%; G=19.9% and C=19.8%

• Termed “Chargaff’s Rules”

A = 30%C = 20%T = 30%G = 20%

Rosalind Franklin 1951 (England)

• Worked on x-ray crystal diffraction photographs (how light bounced off crystals) of DNA structure

• Worked in same area as Maurice Wilkins who tended to treat her as his assistant– Wilkins was working on separate

DNA projects

Structure

• … was still unknown– How did the phosphate, deoxyribose, and

4 nitrogenous bases fit together?

James Watson and Francis Crick, 1953 (England)• Wilkins showed Watson Dr. Franklin’s x-ray

photograph without her permission• Watson said "The instant I saw the picture, my

mouth fell open and my pulse began to race.”– Showed photo to Crick who

instantly began work with Watson on DNA model using balls and sticks• 1953 scientific paper

published called “A Structure for Deoxyribose Nucleic Acid”

Nobel Prize 1962

• Maurice Wilkins, Francis Crick, and James Watson received Nobel Prizes for Physiology or Medicine for work on DNA structure– Rosalind Franklin died 4 yrs earlier in 1958 at age

37 of ovarian cancer– Nobel prizes are only given to living persons so she

will never be honored

MOVIE – SECRET OF PHOTO 51

• Meet the unsung heroine behind the discovery of DNA’s double helix.

Rosalind Franklin

1920-1958