The Story of DNA (9.3) Flashcards
The Griffith Experiment
1928: Frederick Griffith
Was a bacteriologist trying to develop a vaccine for pneumonia
Non-virulent strain: II-R (rough)
Virulent strain: III-S (smooth)
Mice died when injected with live R (non virulent) bacteria plus heated killed S (virulent) bacteria because the R bacteria had taken up the virulence factor from the dead S bacteria and transformed into virulent bacteria
From the results, you can conclude that something in the smooth strain can convert the rough strain into a virulent strain
Also from results, injecting heat-inactivated smooth strain is not a good vaccine strategy for preventing pneumonia
Griffth claimed that there must be a “transforming principle” that converted pseudomonas from rough to smooth
Avery-Macleod McCarty Experiment
1944: Oswald Avery-Colin MacLeod-Maclyn McCarty
Avery, MacLeod, and McCarty repeated the Griffith Experiment, adding enzymes (proteases) that could degrade specific substance to the mixture prior to injection
Rough strain + heat inactivated smooth strain + no enzyme, or protease, or RNAses, DNAses, or lipases, or polysaccharides
All mice died except for DNAse
Found that the transforming principle was DNA
The Avery-McLeod-McCarty Experiment was met with criticism
Thought proteins were more complicated than DNA and that they were likely the important hereditary material
Research with radioactive materials. Proteins contain _____. DNA contains ______
Sulfur; phosphate
Alfred Hershey and Martha Chase
1952, Cold Spring Harbor
Hershey and chase were virologist studying bacteriophage
They knew that phage was simple and comprised only of protein and DNA (no lipids, carbohydrates)
Hershey-Chase hypothesis
Either protein or DNA is the heritable (transforming) material
Bacteriophage life cycle
Step 1: attachment
Step 2: penetration
Step 3: transcription
Step 4: replication of phage DNA and synthesis of proteins
Step 5: assembly
Step 6: release
Hershey and Chase created viruses with radiolabeled proteins (35S) or DNA (32P)
If protein was the transforming principle then 35S will be in the infected cell
If DNA was the transforming principle then 32P would be in the infected cell
Great idea but, how can we separate the virus from the cell so that we can “see” what is on the inside?
The waring blender experiment
–>supernatant= free virus
–>Created pellet=cell
If DNA was the transforming principle, the radioactivity
would be in the pellet
If protein was the transforming principle, the radioactivity
would be in the supernatant
What were the results with the radiolabled proteins and DNA and the waring blender experiment?
When phage coat protein labeled with S35, radioactivity was in the supernatant
When DNA labeled with P32, radioactivity was in the pellet
Chargaff Experiment
Discovered that A=T and G=C
Forms of DNA
A DNA: more compact, major groove, where nucleotide bases are more accessible to DNA binding proteins, allowing DnaA to bind to a specific sequence, favored under low water conditions ds RNA, some DNA/RNA hybrids
B DNA: double helix
Z-DNA: less compact, transient structure, no major/minor groove, favored under high (dGC)n, high salt, induced by negative supercoiling
Rosalin Franklin
X-ray crystallographer
Contributions
–>Coal and graphite structures
–>DNA structure
–>Virus structures (polio virus)
DNA replication: Meselson and Stahl Experiment
1958: Meselson and Stahl
Their idea was to grow bacteria in 15N so that all DNA is “heavy” then shift to 14N (light)
What conformation is DNA structure usually found in?
“B” double helix
