1. SCIENCE AS A PROCESS WHAT DO WE KNOW ABOUT DNA?
HOW DID WE FIGURE IT OUT?

2. Archibald GARROD (1909)
1st to suggest genes dictate phenotypes through enzymes that catalyze specific reactions in cells
Coined term “inborn errors of metabolism”
Hypothesized that patients with alcaptonuria lack an enzyme in metabolic pathway
ALCAPTONURIA-
“Black urine” disease
Lack enzyme to break down alcapton
Builds up in joints (osteoarthritis), heart valves, and kidney stones
Excess excreted in urine

3. Frederick GRIFFITH (1928) See the experiment
Bacterial transformation of pneumonia bacteria in mice

4. Oswald AVERY, Colin MACLEOD, and Maclyn MCCARTY (1944)
Repeated Griffith’s experiment but added enzymes to destroy different kinds of molecules.
. . . DNA is the transforming molecule

5. George BEADLE and Edward TATUM (1940’s) “one gene-one enzyme”
Neurospora bread mold mutants
Each lacks different enzyme in biochemical pathway
If add missing substance,
mold is able to grow
Experiment images from: Campbell and Reece AP Biology

6. Alfred HERSHEY and Martha CHASE (1952)
See their experiment
DNA = genetic code molecule

7. ROSALIND FRANKLIN and MAURICE WILKINS
Analyzed DNA with X-ray crystallography to try
and determine its structure
JAMES WATSON & FRANCIS CRICK
used Rosalind Franklin’s X-ray crystallography
images (PHOTO 51) to come up with alpha helix
model for the structure of DNA

8. Matthew MESELSON and Franklin STAHL (1958)
Matthew MESELSON and Franklin STAHL (1958)
DNA copies by semi-conservative model
See their experiment

9. Images from: http://instruct1. cit. cornell
MESELSON & STAHL
Grew bacteria for many generations in radioactive (heavy) 15N so all DNA is heavy
Then grow in 14N, centrifuge as generations divide, and check to see where heavy DNA ends up

10. MESELSON & STAHL Can tell which model it is by the banding
patterns of DNA molecules
SO WHAT ?
Provided evidence for
SEMI-CONSERVATIVE
REPLICATION MODEL

11. Telomeres protect DNA from being degraded
Telomeres become shorter with each replication; shorter in older cells
Telomerase enzyme lengthens telomeres
Stem cells, germ cells making gametes, and cancer cells have increased telomerase activity
2009 Nobel Prize Physiology/Medicine
Discovery of Telomeres
Jack Szostak Carol Greider Elizabeth Blackburn.