THE STORY OF DNA
DISCOVERY OF NUCLEIC ACIDSFriedrich Miescher, he fine the genetic material from white blood cell nuclei. He noted that the acidic nature and called it NUCLEIN
DISCOVERY DNA COMPONENTSPhoebus Levene - Determined the components of DNA,
They are adenine,guanine,thymine,cytosine and deoxyribose phosphate.
He proposed that four nucleotides per molecule make the simple structure.
DNA could not store the genetic code because it was chemically far too simple.
But theIr propous are incorrect and he dies before the important discovery.
FEDERICK GRIFFITHHe was the firts person that demostrate bacterial transformation.
In the experiment he used two strains of streptococus
1. called type S.S - virulent (deadly)
2. called type R - non-virulent (harmeless)
GRIFFITH'S EXPERIMENTThe experiment he used mouse to discovery
The first is to put the rought (non-bacterial), and the mouse lives, but when he puts the smooth strain in the mouse, the mouse dies.
If you put the heat-killed smooth strain, you kill the bacteria fists and the mouse lives.
The finally experiment is that you mix the rough strain and the heat-killed, and the mouse dies, because the rought strain has a transformation in some way. The question is: How did that happen?
Griffith not resold the question
THE QUESTION - HOW DID THAT HAPPEN?This question is answered by tree people:
1. Oswald Avery
2. Colin MacLeod
3. Maclyn McCarty
PROCESS EXPERIMENTThey mix Rought strain + Heat-Killed smooth whit Protease and the mouse dies
They mix Rought strain + Heat-Killed smooth whit DNase and the mouse live The conclusion is that DNase is the responsable for the transformation. This experiment wish publish in february, 1944
Avery, MacLeod and McCarry EXPERIMENTIn this experiment explain Federick Griffith's results.
They determined the cause of the transformation For this used the rought strain and the smooth strain, the same as Griffith but with one of two enzymes. 1. Protease which destroy protein 2. DNase which destroy DNA
COUNTING NUCLEOBASESHe looked at different organisms and he simply measured the amounts of the four bears atenine, thymine, cytosine and guanine.
No matter what organism that he looked at, he found the fowing trends.
ERWIN CHARGAFFErwin emigrated to the United States during the nazi era. And he became a professor of biochemistry.
He is interested in the percentage of the nucleobases.
He used the paper chromatography and UV spectroscopy to examine the abundance of the nucleolus and he started to notice something very strange.
CHARGAFF’S RULESThe amount of adenine and thymine were always balanced, and the amount of cytosine and guanine, always in balance.
This is a big discovery, but he himself didn’t realize the importance.
He did, however, share this discovery with Watson and Chick.
EXPERIMENT OF COUNTING NUCLEOBASESTo do the experiment he used different animals:
The first is an Octopus and we see that the Adenine and Thymine are almost the same, and with the Cytosine and Guanine are the same too.
The second is a Sea Urchin, he got the same result with A-T and C-G
The third is a rat and the same results.
Finally use a Grasshopper, same thing.
He experiment with humans and the same think.
RESULTS EXPERIMENT DNALabel the DNA with P32 radioactive phosphorus, and then centrifuge and sea where the radioactivity ends up.
Now all the radioactivity is in the pallet. It's all in the bacteria. There’s no fluid on top.
That we were DNA and not the proteins that was the genetic material.
HERSHEY-CHASE EXPERIMENTThis experiment is with phages.
Bacteriophages are viruses that infect bacteria.
They are made of either DNA or sometimes RNA and the rest is made of proteins. The first experiment they use a bacteria cell, have a nuclear material, and then took bacteriophages labeled one of two ways. With radioactive sulfur and that allowed then to follow the proteins in the phage.
They used radioactive DNA to follow the movement of DNA during the infection
SECOND EXPERIMENTThey took the labeled plages, they would expose them, allow them to infect the bacreins
And them would separate what was in
This surprised a lot of people.
To label the proteins, they used radeo-labeled sulfur S35. Now they can see what happens with the phage.
When the phage infests the will disengage. No radioactive material inside the pellet It was all in the supernatant. All in the fluid outside
Anything that was protein from the phages did not get into bacterial cells.
DNAThe race was on to determine the structure of DNA in cells and to determine how it codes for proteins and how it replicates.
They are a problem: DNA exists in tree from:
A form (dry form)
B form (wet foam, as DNA exist inside in cells)
C form (mix the A and B form) Create a question: What does this DNA look? How does it work?
Unfortunately, there was a lot of confusion about this.
FIRST MODELWatson and Crick wrote a paper in which they described DNA as a double
Helix with segers amor phospeaters at the center and the nucleobases fencing the outside.
This model was quickly shown incorrect and in fact it made no chemical sense. All those negatively charged phosphates on the inside would have nea pretty messy
ROSALIND FRANKLINThis girl was an x-ray crystallographer, she talks abaout amazing photographs of the B form of DNA. She figured out how to see the wet form, the form that exists in cells. One of the most famous that she gave is a cooled photo 5. This photo shows the X, which is a sign of a double helix.
The image went to Maurice Wilkins and he communicated this to Francis Crick and James Watson.
FRANCIS CRICK AND JAMES WATSONWhen they have the photo, they know exactly what it means and they know that the model in 1951 was backwards.
The model builders built the model based on Rosalind’s image and we have the paper that most people now that describes what DNA looks like.
Francis Crick, James Watson and Maurice Wilkins, they have all the importance of the work and Rosalind Franklin no have a recognition.
DNA FINALLYNow we know that DNA is a double-stranded helix with the backbone make of sugar and phosphate groups running anti-parellel. Hydrogen bonds between the nucleobases, A with T and G with C. That explains why they are always in the same amounts.
The sequence of nucleobases is what codifies the sequence of amino acids in a protein.
TRIPLE HELIX?Linus Pauling Discovering the structure of the alpha helix and the beta sheets in proteins, he came up with a triple helix model, again. With the phosphates and the sugar on the inside and the nucleobases outside. He was mostly certainly looking at X-ray crystallography images, that were mixtures of both the A and B form. This turned out to bé incorrect.