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 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.
- 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).