SCIENCE AS A PROCESS. http://profiles.nlm.nih.gov/CC/A/A/A/Z/_/ccaaaz~.jpg. WHAT DO WE KNOW ABOUT DNA? HOW DID WE FIGURE IT OUT?. http://en.wikipedia.org/wiki/Rosalind_Franklin. http://www.mun.ca/biology/scarr/Chase_&_Hershey_1953.jpg.
WHAT DO WE KNOW ABOUT DNA?
HOW DID WE FIGURE IT OUT?
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
“Black urine” disease
Lack enzyme to break down alcapton
Builds up in joints (osteoarthritis), heart valves, and kidney stones
Excess excreted in urine
Bacterial transformation of pneumonia bacteria in mice
Repeated Griffith’s experiment but added enzymesto destroy different kinds of molecules.
. . . DNA is the transforming molecule
Experiment images from: Campbell and Reece AP Biology
Neurospora bread mold mutants
Each lacks different enzyme in biochemical pathway
If add missing substance,
mold is able to grow
DNA = genetic code molecule
Analyzed DNA with X-ray crystallography to try
and determine its structure
1953 - 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
DNA copies by semi-conservative model
Images from: http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/106/unit01/6.dnareplicationmodels.htmlMESELSON & 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
Can tell which model
it is by the banding
patterns of DNA molecules
SO WHAT ?
Provided evidence for
Telomeres protect DNA from being degraded http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/106/unit01/6.dnareplicationmodels.html
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
Discovery of Telomeres
Jack Szostak Carol Greider Elizabeth Blackburn.