Story of DNA Flashcards
What did Hunt know?
Knew about genes + Linkage + chromosomes
Knew chromosomes contain genes and they are hereditary material BUT they didn’t know what they were made of
Griffith experiment (overall)
Transforming Principal –> Griffith claimed there must be a “transforming principle” that converted Pseudomonas from rough to smooth
Griffith experiment
Transforming Principle
Griffth = was a bacteriologist trying to develop a vaccine for Pnuemonia
- Had a strain of Step Pnumoniae (causes Pnumonia)
Strains:
1. Non-viraent R (rough) –> Mouse doesn’t die
2. Virulent (Smooth) –> mouse dies
Griffith Experiment results
Heat kills smooth strain – virulent strain but killed first
Rough – can get non-virulent strain from alive mice
Smooth – get strain from dead mouse
Heat killed smooth –> get no virus from mouse
Rough strain and heat killed smooth strain –> Can get virulent smooth strain from mouse
If have strep not causing virus and inject self with virulent = will turn non-virulent in body to virulent
Means that this is a bad vaccine strategy
What are possibilities to cause Transforming principle
DNA
Proteins
RNA
Lipids (Fats)
Polysaccarides
They knew the transforming principle happened + knew these molecules existed but didn’t know how they work together
Research in great depression
Research into DNA was stalled due to the Great Depression – no money to do research
Oswald Avery Colin + Macleod-Maclyn Mccarty (overall)
Looking at what causes Transforming Principle
They repeated Griffth’s experiment + they added enzymes (proteases) that could degrade specific substances to mixtures prior to injections
- They took heat killed smooth + rough + dofferent enzymes (one added proteases; one added RNAs etc.)
Results of Oswald Avery Colin + Macleod-Maclyn Mccarty experiment
Result: DNA prevents Smooth from converting rough to smooth = DNA is transforming principle
Showed DNA = what the transforming principle is == only one where when we take it away (when degrade it) the mouse lives
Critism in Oswald Avery Colin + Macleod-Maclyn Mccarty experiment
Overall – “HOW could 4 nucleotides explain ALL heredity”
- Proteins are more complicated than DNA and proteins do stuff so it was though that special proteins were likely the important hereditary material
- No one believed them because biochemists knew Amino Acids make proteins –> Knew DNA only had 4 nucleotides – thought 4 nucleotides was way to simple to cause the Transforming principle = thought it was protein
- People believed Amino Acids were what was important
Criticism:
1. Your DNA preps are not pure –> 1% of proteins still present even the best of DNA preps
Issue with criticism = bas to make a conclusion then find science that supports it
What did biochemists show (during DNA story)
At the time there was a lot of research with radioactive materials
Shows:
1. Proteins have sulfar; DNA does not
2. DNA has phosphate groups; proteins do not
BOTH have Nitrogen
Bacteriophage Cycle
Phage sits on E.Coli and injects something in cell and virus does not actually enter cell
- There is something in cell that makes more cirus
Alfred hershey + Martha Chase Experiment (Overall)
Hershey + Chase = were viroligists studying bacteriaphage
Knew that phage was simple + comprised only of protein and DNA (no lipids or carbs)
- Knew enough to transform something –> could replicate virus with something
Hypothesis – Either protein or DNA is the heretible material
Use of bacteria Phage
Good system to figure out if hereditary material is DNA or protein
Hershey + Chase experiment
Used radiolabeled Proteins and DNA
Labeled:
Proetins – 35S –> Only thing that has sulfar
DNA – 32P – only thing with phosphorous
- Virus took radiolabeled DNA
If proteins was the Transforming principle the s35 would be in the cell (radioactive would be in the supernatenet)
- Have rediolabel in celll if proetins ate labeled
If DNA was the Tranforming principle then P32 would be in the infected cell (radioactivity would be in pellet)
- Only radioactive in cell if have radioactive DNA
Results:
When phage has radioactive sulfar –> Radioactivity was in the supernatenet
When DNA was labeled –> radioactivity was in pellet
SHOWS that DNA was injected into E.coli – allows virus to replicate itself
Blender experiment
Part of Hershey Chase Experiments
Virus sticks to cell –> could isolate bacteria but radioactive virus will still be on the cell = put in blender
- Blender helps virus detatch from cell
THEN centrifuge –> cells pellet to bottom and the supernatent should have the virus no longer attatched
AFTER blender:
If portein is TP – radioactive in the pellet
If DNA is TP then Radioactive would be in the pellet
What allows Virus to replicate itself
Clear DNA injected into the pellet allows Virus to replicate itself
What happened after discovered DNA was the hereditary material?
The race to solve the structure of DNA began
Before this = started to find the structure of proteins + knew DNA was the hereditary material
Chargaff
Found:
1. Number of As = Numbers of Ts
2. Number of Gs = Number of Cs
Reason = because of H-bonds holding them together
True in all species BUT different species can have different G-C content
How do we find structure of DNA
X-ray Diffraction –> get structure of DNA
Steps:
Take structure –> Blast with X-ray –> Anything that hits DNA/protein is refracted light in different directions –> get seperation of light on image film –> can deduce distance away from the mean
- From parameters you can work backwards to find the structure
Issue with X-Ray diffraction
Get the structure of DNA BUT DNA can take different forms
Issue: DNA is dynamic but X-ray need one static structure
- Initial X-ray pictures were a mess (the result of a mixture of DNA forms in crystals) – couldn’t interpret
- Because DNA can take many forms
Forms of DNA
DNA can take several forms – A, B, and Z
***Need different forms during different times – DNA can be dynamic
A form of DNA
More compact
Favored under low water conditions
dsRNA
Some DNA/RNA hybrids
Z form of DNA
Less compact
Transient structure
No major/minor groove
Favored under high dGC + High salt
Induced by negitive supercoiling
Structure of DNA is…
DNA structure is Dynamic –> Need to unwind to get to DNA
During unwinding = need to change shape of DNA
The race to solve the structure
Rosalind Franklin – X-ray Crystolographer
- Did not get along with Wilkins
Maurice Wilkins – X-ray Crytsolographer
James Watson + Crick – Crystologrpher
- Watson/Crick + Pauling –> were talking in Mens club – they all wanted to get the answer before Linnus Pauling
Linnus Pauling – Got nobel prize for covalent theory
Linnus Pauling
Trying to find structure of DNA – He built a model and then tried to get the data to match the model
Model = Triple elix
Rosalin Fraklin
It was HER image that helped solve the actual structure of DNA
- She was an X-Ray crystolograoher – meticulous
Approach – she would not build model before data –> She wanted to get the data and then build the model – “Purist”
Contirbutions:
1. Coal and Graphite sturctures – Looking at structure if Carbon
2. DNA structure
3. Virus Structures (Polio)
2 Nobel prizes were awarded to others after her death for DNA and Viral Structures
Wilkins model
First double helix model BUT the bases point outwards
- Nucleotides face outwards
***Made Rosolin mad because this did not match the data
Wilkins + Rosolin Relationship
They did not get along
Wilkins thought Rosalion would be a teachnition BUT she thought she would be equal
- She got mad at Wilkins about his model because the data did not match the model he built
Photo 51 (Story)
Roslin took image – Wilkins saw the image and brought it to the mens club and shared it with Watson + Crick
After seeing the pictire Watson/Crick realized that this meant that DNA has double helic were the nucelotides face inwards – solved the structure once they saw the picture
They solved the structure after they saw the picture – From this image W/C new that DNA was a double helix (not triple) and they knew the distance between the bases and that the bases were pointing inwards
W/C model
After seeing the picture – built a new model that had the bases pointing inwards stacking together in wats that were consistent with Franklins Photo
- They published the model without Photo 51
- Rosolin saw the paper and asked where the data was –> She left Wilkins lab because she was so fed up
W/C got all of the credit
Rosolin after picture
Left Wilkins lab + She published work but it was placed at the end of the journal + lacks novelty
- She did publish her work – including the location of water molecules in the same issue but it was put after the W/C paper and lacked notoriety
W/C got all of the credit
THEN Franklin moved to a new instatute where she worked in the structures of RNA viruses and polio virus
She dies of Ovarian Cancer – Hypothesized that it was due to radiation damage + Ashkenazi Jew
Sexism in Science
W/C + Wilkins did not acknowledge contrubution of Roslin Franklin BUT W/C + Wilkins got nobel prize
Watson was…
SEXIST – made many sexist + Racist + Abelist commensts and never apologized for his beleifs
- Wouldm’t hire someone that was overweight
- Thought women shouldn’t work in a lab with men
- Promoted genetic screening to increase buety + inteligence + Behaviores (Eugenics)
- Proposed that all traits were entirelly genetiocs and linked to different races
He was forced to retire from Cold Spring Harbour + CSHL tittles revoked
What did DNA strcuture provide
The Structure of DNA provided a way to understand how DNA can be copied during inheritance
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for genetic material”
Overall Timeline
DNA Structure
Typically found in the B double helix conformation
10 BP/turn
Has major + Minor grooves
Use of Major/Minor Grooves
Major and Minor grooves provide protein recognition sites
Bind to major – Sequence specific DNA binding proteins typically bind to major groove
- Example = Transcription Factors
Bind to minor – Non-specific DNA Binding proteins typically bind to minor groove
- Example – Histones
Where does transcription occur
Eurochromatin – regions of transcriptional activity
Shows chromatin can be remodeled
Example – Chromosomal puffs in Drosphilla show states of de-condensing in regions being actively transcribed
