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DISCOVERY OF NUCLEIC ACIDS (FREDERICH MIESCHER)
-Insulated genetical material from white blood nuclei. It had an acid nature called nuclein -
DISCOVERY OF DNA COMPONENTS (PHOEBUS LEVENE)
-Determinate the components of DNA: adenine, guanine, thymine, cytosine and deoxyribose phosphate.
-Defined phosphate, sugar and base units called nucleotides -
LEVENE'S TETRANUCLEOTIDE (PHOEBUS LEVENE)
-He proposed that there were 4 nucleotides per molecule
-DNA could not store the genetic code because it was chemically far too simple -
Griffith Experiment 1 (FREDERICK GRIFFITH)
What does he do?
He used 2 strains of Streptococcus
- Type S: virulent (deadly) S is for Smooth, that is made of a capsule which make the bacteria virulent and deadly, so they cause the mice to die,
- Type R: non-virulent (harmless) R is for Rough, because they don’t have that smooth so mice, when it’s exposed the do just fine. -
Griffith Experiment 2 (FREDERICK GRIFFITH)
How it works?
There are 4 options:
→ If you take the rough non-virulent strain, injected into a mouse, no problem.
→ If you take the virulent strain, injected into a mousse, the animal dies.
→ If you heat-kill the smooth strain, so you kill the bacteria first, the mouse will live.
→ If you mix mixed the rough strain and the heat-killed smooth together, the mouse dies.
So, there’s a transformation that took place, the rough strain was transformed in some way by the heat-killed smooth strain. -
Griffith Experiment 3 (FREDERICK GRIFFITH)
How did that happen?
This question was answered by three people: Oswald Avery, Colin MacLeod and Maclyn McCarty. Their experiments in 1944 explained Frederick results. -
JOURNAL OF EXPERIMENTING MEDICINE 1 (OSWALD AVERY, COLIN MACLEOD AND MACLYN MCCARTY)
They determined what caused the transformation and they figured this out by taking the live rough and the heat-treated S, just the same as Griffin had done, but the mixed them with one of two enzymes. They did two options, the first one was mixed with a protease. A protease destroys proteins. And the second, was mixed with DNase which destroys DNA. So, at the first option, the mouse dies and in the second, he doesn’t die. -
JOURNAL OF EXPERIMENTING MEDICINE 2 (OSWALD AVERY, COLIN MACLEOD AND MACLYN MCCARTY)
That means that DNA was responsible of transformation because if you chop up the DNA with the DNase it’s not virulent any more.
-This really big discovery was published the same year and here they suggest that it is DNA and not protein that may be the hereditary material of bacteria and the propose perhaps in higher organisms as well -
DOUBLE HELIX (WATSON AND CRICK)
- They described DNA as a double helix with sugars and phosphates at the centre and the nucleobases facing the outside. This was incorrect, it made not chemical sense. All those negative charged phosphates on the inside would have been messy and the thing would probably have exploded with negative charges. They throw in some catines calcium and magnesium, but there was absolutely no data to support this.
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COUNTING NUCLEOBASES 2 (ERWIN CHARGAFF)
-He noticed the amounts and the adenine and thymine were almost the same and the amount of cytosine and guanine were the same.He tried with other animals:sea urchin, a rat, a grasshopper and humans. And the results were the same.This came to be known as Chargaff’s rules.The amount of adenine and thymine were always balanced and the same with cytosine and guanine. This was a big discovery but he himself didn’t realize the importance of these findings.He shared his discovery with Watson and Crick. -
PHOTO 51 FRANCIS CRICK JAMES WATSON MAURICE WILKINS AND ROSALIND FRANKLIN
Rosalind took a lot of amazing photos of the B form of DNA, she figured out hoe to see the wet form before that exists in cells. The photo 51,it clear shows the sign of double helix. But she was not prepared to publish it. But Maurice Wilkins, got the photo from her desk in king’s college and managed to get into Watson and Crick in Cambridge. When they saw the image, they knew exactly what it meant, and they knew that their model from 1951 was backwards or inside out. -
COUNTING NUCLEOBASES 1 (ERWIN CHARGAFF)
He started too count nucleobases also, he started to notice something very strange:
- He looked at different organisms and he simply measured the amount of the four bases: adenine, thymine, cytosine and guanine and no matter organism that he looked at he found the following trends (example of an octopus):
ORGANISM: Octopus
% ADENINE: 33.2
% THYMINE: 31.6
% CYTOSINE: 17.6
% GUANINE: 17.6 -
ALFRED HERSHEY AND MARTHA CHASE
- They did amazing experiments going on with phages.
- Bacteriophages are virus that affect bacteria. They are made of either DNA or sometimes RNA, and the rest oh the phage is made of proteins. So that’s going to make up the head and the tail, the tail fibers, kind of looks like a lunar lander.
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ALFRED HERSHEY AND MARTA CHASE 2
-They used the bacteria cell and then they took bacteriophages labelled one of two ways. Either with a radioactive sulphur and allowed them to follow the proteins in the phage or they used radioactive DNA to follow the movement of DNA during the infection. -
ALFRED HERSHEY AND MARTA CHASE 3
What do they do?
They took one of the labelled phages. They would expose them to infect the bacteria, and the they would separate what was in the bacteria from what was not in the bacteria by centrifugation. You spin the tubes fast and what you get is the supernatant, which is the fluid on top everything outside the cells and in the pellet, you get the compressed bacterial cells and everything that’s inside. -
ALFRED HERSHEY AND MARTA CHASE 4
How was it done?
EXPERIMENT 1: To label the proteins they used radio-labelled sulphur S35. Sow they’re going to be able to follow the proteins in the phage and see what happens. They allow them to infect, and then the phage will disengage. The result was no radioactive material inside the pellet. It was all in the supernatant. All in the fluid outside. So, anything that was protein from the phage did not get into the bacterial cells. -
TRIPLE HELIX (LINUS PAULING)
- He came up with a triple Helix model with the phosphates and the sugar on the inside and the nucleobases outside. He was more certainly looking at an x-ray crystallography images that were mixtures of both the A and B form. That turn out to be incorrect.
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ALFRED HERSHEY AND MARTA CHASE 5
EXPERIMENT 2 So, label the DNA with P32,radioactive phosphorous and allow the pages to infect,just like before, and then centrifuge and see were the radioactivity ends up,and now all the radioactivity is in the pellet,it’s all in the bacteria.There’s no radioactivity in the fluid on top. CONCLUSIONS:DNA was not a protein, was the genetic material.A protective protein coat was formed around the bacteriophage,but the internal DNA is what you conferred its ability to produce progeny inside bacteria -
ACTUAL DNA
- The backbone is made of sugar (deoxyribose) and phosphate groups.
- Hydrogen bonds between the nucleobases: A-T and G-C
- The sequence of nucleobases codifies the amino acid sequence of a protein Strings of base pairs that code for a product are called genes
