Things about DNA you won't find in books

Recalling The History Of DNA Discovery

Today I want to talk about DNA and the history of the discovery of this molecule which will hopefully give you some context into which you can put all the modern discoveries in molecular biology, the human genome, bioinformatics and so much more.

 So we're going to take a step back and start back in the mid-1800s that's actually when nucleic acids were discovered and shortly thereafter Phoebus Levine discovered all of the components of DNA and actually defined the unit including the sugar, the phosphate and the base which he called a nucleotide that he got correct. Unfortunately he didn't quite get this structure he actually thought it was organized in tetrads he said there were four nucleotides per molecule and that this made such a simple structure that it couldn't possibly be the key to unlocking heredity.

 Now unfortunately he was incorrect on both of those counts. He died in 1940 before the importance of his work was really fully understood. But he was a major contributor.

 Frederick Griffith actually was a bacteriologist who studied two strains of streptococcus that caused pneumonia and he was the first person to actually demonstrate bacterial transformation.

Griffith's Experiment:

Griffith Experiment

So let's take a look and see what he did. He used two strains of streptococcus one that is called type S (S for smooth). These smooth colonies make a capsule which make the bacteria virulent and deadly so they caused the mice to die. There is also a strain Type R which is non virulent or harmless. (R is for ruff) so they do not have that smooth capsule and mice when exposed to it they do just fine.

 So here's how it works. If you take the rough non virulent strain inject it into a mouse no problem. If you take the smooth strain the virulent strain inject that into a mouse, mouse dies. If you heat kill the smooth strain so you kill the bacteria first the mouse will live. If you mix the rough strain and the heat killed smooth strain 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. The question was how did that happen? Now Griffith did not figure this out.



 This question was answered by three people, Oswald Avery, Colin MacLean and Maclin McCarty and their experiments in 1944 explained Fredrick Griffiths results. They determined what actually caused the transformation and they figured this out by taking the live rough and the heat treated S just exactly the same as Griffith had done but they mixed them with one of two enzymes one group was mixed with a protease. A protease destroys protein the other was mixed with a DNA which destroys DNA.

Avery and group's Experiment:

So here's what they did they took the nonvirulent strain they mixed it with the heat killed smooth virulent strain one group got the protease inject into a mouse the mouse died.

 The other group was mixed with the DNA injected into mice. The mice lived. So this showed that it was the DNA that was responsible for the transformation because if you chop up the DNA with the DNA’s it's not virulent anymore.

 This was a really big discovery. It was published in February 1944 and here they suggest that it is DNA and not protein that may be the hereditary material of bacteria and they proposed perhaps in higher organisms as well.

 So now we have the DNA discovery timeline with the structure of nucleic acids, what DNA is made of. Griffith who did the first transformation and then Avery, McLean and McCarty who explained the mechanism of that transformation and now things are really going to start to heat up.

Entry of Chargaff into DNA research:

There was someone who started counting nucleobases his name was Edwin Chargaff. Chargaff emigrated to the United States during the Nazi era and he became a professor biochemistry at Columbia University Medical School and he actually was interested in percentages of the different nucleobases and he started to notice something really strange.

 He looked at different organisms and he simply measured the amounts of the four bases adenine, thymine, cytosine and guanine and no matter what organism that he looked at, he found the following trends that the amounts of adenine and thymine are almost the same the amount of cytosine and guanine is almost the same.

 Well that's odd he looked at a sea urchin and he got the same thing A and T about the same, C and G about the same. A rat guess what the same thing and a grasshopper same thing and this is really pretty bizarre oh he looked at humans too of course and he found exactly the same result.

 This came to be known as Chargaff's rules. The amount of adenine and thymine were always in balance and the amount of cytosine and guanine always in balance.

Now this was a really big discovery but he himself didn’t actually realize the importance of these findings. He did however share his discovery with Watson and Crick at the Cavendish lab in Cambridge in 1952 and Watson and Crick actually knew what it meant. Chargoff was actually left out of all the big recognition of the discovery of DNA and after the Nobel Prize was awarded which he got no pardon he became kind of a recluse and spent the rest of his life writing to scientists about why he was excluded.

Hershey and Chase's Discovery:

Meanwhile Hershey and Chase had some pretty amazing experiments going on with phages.  Bacteriophages are viruses that infect bacteria. So we call them a bacteriophage or just a phage for short. They are made of either DNA or sometimes RNA and then the rest of the phage is made of protein.
So that's going to makeup the head and a tail the tail fibers looks like a lunar lander. So what Hershey and chase did was they used a bacterial cell there's a genome in of the bacteria, the bacteria of course has its own nuclear material that's going to be the host and then they took bacteriophages labelled one of two ways either with radioactive sulphur and that allowed them to follow the proteins in the phage or they used radioactive DNA to follow the movement of DNA during the infection.

Proof that DNA was the genetic material:

Hershey And Chase Experiment

 Here's what they did. They took the labelled phages they would expose them, allow them to infect the bacteria and then they would separate what was in the bacteria from what was not in the bacteria by centrifugation.

 Right so you spin the tubes really fast and what you get is the supernate 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.

 So here's how it was done. To label the proteins they used radio labelled sulphur s35. So now 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. What about the DNA?  So label the DNA with p32radioactive phosphorus and allow the phages to infect just like before and then centrifuge and see where 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. This came as a big surprise to a lot of people and they were very hesitant in the way they said it.

But Hershey and chase concluded that it was the DNA and not the protein that was the genetic material and that the only real need for the protein was to sort of serve as packaging you know to cover the thing so it's the DNA.


 Now there’s a mountain of evidence saying that it's the DNA that's the genetic material, but the question was what does this DNA look like and how does it work. Unfortunately there was a lot of confusion about this because DNA exists in two forms the A form which most people were looking as it was easier to get. And then the B form which is actually what DNA really looks like inside cells.

 Introduction Of First Structure Of DNA:

Watson and Crick

 A lot of people were looking at a mixture of the two. Some famous model builders Watson and Crick came up with their first model in 1951 and they described DNA as a double helix with sugars and phosphates at the centre and the nucleobases facing the outside.

This was incorrect made absolutely no chemical sense. All those negatively charged phosphates on the inside would have been pretty messy and the thing probably would have exploded with negative charges.

 But that's okay they threw in some cations calcium and magnesium there was absolutely no data to support this that was actually kind of an embarrassment. 

Linus Pauling who discovered the structure of the Alpha helix YZ 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 on the outside. He was most certainly looking at x-ray crystallography images that were mixtures of both the A and B form.

Triple Helical DNA structure

 If  you look at images like that you really do see something that looks almost like a triple helix that turned out to be incorrect. we have three people in the history, Francis Crick James Watson they were working at the Cavendish labs in Cambridge and Maurice Wilkins and someone you may have never heard of Rosalind Franklin at King's College in London.

Who Is Rosalind Franklin?

Rosalin Franklin

 Rosalind Franklin was an x-ray crystallographer who really didn't get along well with anybody especially not with Wilkins and what happens next there’s a lot of debate and books written about it. Story kind of goes like this.

Discovery of Rosalind Franklin:

Rosalind took a lot of amazing photographs of the B form of DNA she figured out how to see the wet form the form that exists in cells this is probably the most famous image that she got they call it photo 51 and this photo shows very clearly the X in the middle that is the sign of a double helix.

Rosalind was a stickler for detail and she was not prepared to publish this until she finished all her calculations but somehow Maurice Wilkins got photo 51 from her desk in King's College and managed to get it to Watson and Crick in Cambridge now when they saw the image they knew exactly what it meant and they knew that their model from 1951 was backwards or inside-out and the model builders built the model based on Rosalind's image and we have the paper that most people know where they describe what DNA looks like.

 A lot of people are not aware that three papers were published one right after the other in the April 25th 1953 edition of the journal Nature. The first one was the Watson and Crick paper. The second one was by Stokes and Wilkins. The third was by Rosalind Franklin and her assistant Raymond Gosling.


The reason why Rosalind Franklin did not share in the Nobel Prize was because unfortunately she had already passed away and Nobel Prizes are only given to living people. Almost more disappointing though is that neither of the three, acknowledged the importance of her work in their discovery and for that I believe there really is no excuse many many people have come to think that they were the ones responsible for the discovery because their article was listed first and that Franklin was merely collaborating that could not have been farther from the truth.

 Rosalind died at the age of 37 from ovarian cancer and it's believed that her exposure to x-rays from all of her work was most likely a contributing factor.

Conclusion:

 She contributed not only to the DNA story but she was a huge contributor during the war effort making a more effective gas masks. She worked on coal and she worked on the structure of viruses as well.

 I think James Watson kind of said it himself in this quote in 1999 you can read it for yourself if you read the different accounts of what happened to the DNA story you get pretty different view of what actually happened. But I kind of let Watson speak for himself here so now you can see how all of these experiments kind of lined up and ultimately led to the correct double helix structure for DNA. Watson and Crick certainly Oh many other people a debt of gratitude but they are generally the ones who get the credit.


Now we know that DNA is a double-stranded helix with a backbone made of sugar and phosphate groups running anti parallel hydrogen bonds between the nucleobases A with T and G with C that explains why they are always in the same amounts and the sequence of nucleotides the sequence of amino acids in a protein it is a truly amazing structure and the history of its discovery I think equally amazing as always.


READ ALSO:

1.  Basics To Genetics
2. What Is DNA And DNA Polymerase

External Reference:

• Wikipedia- What Is DNA 
• Discovery of DNA Double Helix - Nature.com