“What is DNA?”

• It could be found in almost all cells

• It contained phosphorus in the form of phosphate.

What was known about DNA?

• It contained a sugar, deoxyribose.

• It contained 4 forms of nitrogen containing bases; purines - adenine and guanine and pyrimidines - thymine and cytosine.

What was known about DNA?

“#hat is the structure of DNA?”

Maurice Wilkins

Kings College, London

Rosalind Franklin

James Watson and Francis CrickCambridge College, London

Linus PaulingChemistry at California Institute of Technology

(Caltech)

Nucleotide

nucleotide monomer(sugar, phosphate and nitrogen

base A, T, C orG)

nucleotide polymer(chain)

Erwin Chargaff calculated a “rough

equivalent” between adenine and thymine

molecules and between cytosine and guanine

molecules.

adenine guanine thymine cytosine

X-ray CrystallographyRosalind Franklin’s photo 51

right -handdouble helix

Complementary Base PairingC-G and A-T

Watson and Crickwith their model of DNA

double helix 1953

• enzymes separate or unzip the two strands (breaks the H bonds)

• the two unwound strands each become a template for the new strands to be built

• the new nucleotides are paired and inserted

• enzymes “proofread the new strands and seals the base pair

DNA Replication or Synthesis