Discovery of DNA and Its Function

What carried the genetic instructions? Protein or DNA ?

• Proteins seemed best bet because they're complex and diverse.
• DNA little known, but seemed too uniform.

Griffith (1928)

• Griffith experiments with smooth and rough coat Streptococcus pneumoniae.
• Smooth polysaccharide coat strain is pathogenic - kills mice.
• Rough strain doesn't kills mice.
• Heat-killed smooth strain doesn't kills mice.
• A mixture of heat-killed S and R strain injected in mice killed them?!
• Live smooth strain found in the little bodies.
• Transformation - bacteria can assimilate external genetic material.

Chargaff (1949)

• Discovered that ratios of A, G, C and T in DNA varied from species to species.
• Amount of A = amount of T, G = C.
• Suggested DNA maybe more complex than thought!

Hershey and Chase (1952)

• Studied virus that infected bacteria (bacteriophages).
• Phages composed of protein and DNA and can take over bacteria to make more phages.
• Phage proteins contain sulfur but not phosphorus.
• DNA contains phosphorus but not sulfur.
• Grow one group of bacteria and phages with ³⁵S.
• Grow another group of bacteria and phages with ³²P.
• Separate phages, infect new bacteria colony, see if they have ³⁵S or ³²P.
• Infected bacteria had ³²P. So DNA is the genetic material.

Watson and Crick, Franklin (1953)

• The key : Rosalind Franklin's X-ray crystallography image of DNA.
• A regularly-shaped crystal will disperse X-rays in a regular pattern.
• Then math and experience allows experts to deduce structure.
• Her X-ray image suggested DNA had :
  • 2 nm diameter.
  • nitrogenous bases stacked 0.34nm apart.
  • helical structure with full turn in 3.4nm.
• 1953 - Watson and Crick publish one-page paper.
• 1958 - Franklin dies of cancer at age 38.
• 1962 - Watson and Crick, Wilkins receive Nobel prize.

Nucleotide base pairings creates a pattern unique to each species (each organism).