DISCOVERY OF DNA STRUCTURE
Deoxyribonucleic Acid (a.k.a. DNA)
Why is DNA Important?
• All cellular chemistry is controlled by protein enzymes.
• The instructions for making these proteins is built into the structure of the cell’s
DNA. i.e. The DNA molecule contains a code for building proteins
• DNA Controls the production of proteins within the cell thus it controls all of a
cell’s chemistry.
DISCOVERY OF DNA STRUCTURE
The major players were:
Linus Pauling
Rosalind Franklin
Maurice Wilkins
James Watson
Francis Crick
LINUS PAULING
• American
• Pauling won a Nobel prize in 1954 for his work on the molecular structure of
proteins.
• Pauling was also interested in solving the structure of DNA.
ROSALIND FRANKLIN
• Worked as a research assistant at the Cavendish Laboratory in the early
1950’s
• Expert in the difficult process of x-ray crystallography.
MAURICE WILKINS
• Also expert in x-ray crystallography, & also worked at the Cavendish
Laboratory.
• Wilkins was particularly good at interpreting x-ray crystallographs.
• Had a difficult working relationship with Rosalind Franklin.
JAMES WATSON
• American
• Began working at the Cavendish Laboratory in 1952.
• Early on, suspected that DNA was the “molecule of heredity.” (most others
believed it was protein).
• Watson met Francis Crick and discovered their common interest in DNA.
• Watson & Crick were supposed to be working on other projects, not DNA.
• Irritated Rosalind Franklin by referring to her as “Rosy.”
FRANCIS CRICK
• British
• Began work at the Cavendish laboratory in the late 40’s.
• Also believed DNA to be the “molecule of heredity.”
• Quickly became friends with James Watson.
WATSON & CRICK’S FIRST TRY
• Watson & Crick were anxious to show off their first model of DNA to all of the
staff at the Cavendish.
• Several mistakes with their model were pointed out.
• Rosalind Franklin was particularly harsh in her criticism.
LINUS PAULING’S SOLUTION
• Pauling constructed a three-stranded, coiled model of DNA.
• His model was proven to be incorrect.
THE TRUE STRUCTURE IS DISCOVERED
• Franklin’s excellent x-ray photographs of DNA were instrumental in determining
that the DNA molecule is a double, (not triple), stranded helix.
• Maurice Wilkins confirmed the double-helix structure indicated by Ms.
Franklin’s photographs.
• The molecular components of DNA were already known:
Adenine, Thymine, Cytosine, Guanine, Deoxyribose, and Phosphate.
• It was also known that adenine and thymine molecules were always present in
equal amounts.
• The same is true for cytosine and guanine molecules.
• Francis Crick suggested that in the DNA molecule, adenine always pairs with
thymine and cytosine always pairs with guanine.
• James Watson finally completed the picture by figuring out how all of the
components of the DNA molecule fit together to make a two stranded helix.
DETAILS OF DNA STRUCTURE
Discovery of DNA
• 1869 - Dr. Frederick Miescher – first isolated DNA molecules from white blood
cells left on a surgical bandage. He called the substance “nuclein.”
• It became known in the following years that DNA was present in all cell nuclei,
but it’s function & structure remained unknown.
• 1953 – James Watson and Francis Crick discover the molecular structure of
DNA.
Components of DNA
• “Backbone” of alternating deoxyribose sugar and phosphate.
Four Nitrogen Bases: (see p.329) The nitrogen bases are divided into two
groups based on similarities of chemical structure.
• PURINES
–
Adenine (A)
–
Guanine (G)
• PYRIMIDINES
–
Thymine (T)
–
Cytosine (C)
Nucleotide Structure
(see p.329)
• One nitrogen base groups together with a sugar and phosphate group to make
a “nucleotide.”
Nucleotide Pairing
• Nucleotides pair up in specific ways.
• Adenine nucleotides always pair with thymine nucleotides.
• Cytosine nucleotides always pair with guanine nucleotides.
• A DNA strand is built nucleotide by nucleotide.
• The “complementary” nucleotides pair up, such that the two sugar phosphate
backbones are on the outside and the nitrogen base-pairs are on the inside.
• The entire molecule is coiled to give the final shape of a “double helix.”
Imagine a twisted ladder where the sides are made up of alternating sugar and
phosphate molecules, and the steps are made up of paired nitrogen bases, (A-T,
C-G).