How did a german castle, a food blender and some radioactive sulphur ...
completely revolutionise our understanding of life as we know it?
It's time for some Shed Science.
*Scientific whistling*
This is DNA.
You probably know that DNA has a famous double helix structure and that it's the molecule behind genes.
Coding for everything needed to make an organism.
But how do we know this?
Long before the double helix structure was discovered in 1953,
Biologists had already identified nucleic acids,
the group including DNA, as one of the four major groups of chemicals found in cells
including fats, carbohydrates and proteins.
In 1869, this guy, Friedrich Miescher was working at Tübingen Castle in Germany.
And he was looking at the nuclei of white blood cells
which he got from the pus in used bandages from a local hospital.
He managed to isolate the DNA from all the other messy bits of the cell to produce this:
Pure DNA which he called Nuclein because of where it was found in the nucleus of the cell.
In 1923, scientists developed a dye that stained DNA red and left all the rest of the cell colourless.
And in doing so were able to see, that nearly all of the DNA found in a cell is found in the chromosomes.
Chromosomes were already known to be how the genetic material is inherited from one generation to the next.
So that's it! Sorted. DNA is now the bonafide genetic material that carries all the information for life
And we can now crack on and work out how it works. Right?
If only ...
The problem is that chromosomes are made up of protein as well as DNA so now they had to work out
which of the two was the genetic material.
So let's have a look at the evidence.
Chromosomes contain more protein than DNA.
And proteins have 20 different building blocks or amino acids
as opposed to DNA which only has four building blocks or bases.
And so biologists thought, that proteins could, therefore, be much more complex than DNA could ever be .
And so, they concluded DNA wasn't up for the job and that protein must be the genetic material
wrapped around DNA for structural support.
Along came Frederick Griffith in 1923, who wanted to put protein and DNA head-to-head ...
in an experiment to see which one carried the information and to do this he used the bacteria ...
Or pneumonia. He had two mutants of pneumonia and nasty strain and a nice strain.
When he injected the nasty strain into a rat, the rat died.
When he injected the nice strain into the rat, the rat survived.
What Griffith did was to kill the nasty bacteria by heating it.
Making all the bacterial cells explode; forming this nasty cell mix.
And then he added this nasty cell mix to the rat and it survived. Hooray!
But the clever bit was to add this nasty cell mix to living nice bacteria before injecting it into the rat.
And the rat died.
Then they looked at the dead rat afterwards and managed to recover living nasty bacteria
even though all they had added was living nice bacteria and this nasty cell mix.
Griffith concluded that something in the nasty cell mix was able to transform
living nice bacteria into living nasty bacteria.
And so set about a whodunit, looking for this transforming principle
or the chemical that holds the genetic information.
Using enzymes like the ones in your stomach that digest food,
he was able to digest and therefore destroy each of the major suspect chemicals in turn:
Carbohydrates, proteins and DNA.
When carbohydrates were digested, the nice bacteria were still transformed into nasty bacteria.
so they couldn't be the transforming principle or genetic material.
When proteins, the major suspect, were digested,
the nice bacteria were still transformed into nasty bacteria.
Therefore they could not be the transformation principle.
They could not be the genetic material that everybody at the time thought they were.
And you guessed it: when DNA was digested,
the transformation did not occur and nice bacteria stayed as nice bacteria.
At last. Conclusive evidence that DNA is the genetic material containing all the information and not protein.
For all Griffiths hard work and excellent experimental design,
the biological community rewarded him ...
by not believing him at all and saying that his results were purely as a result of contamination.
Because they were just so stuck on the idea that protein - not DNA - must be the genetic material
Viruses to the rescue!
Alfred Hershey and Martha Chase believed Griffith ...
and decided that they could prove him right by studying the viruses that infect bacteria called bacteriophages
or phages for short.
Phages, like other viruses, are really simple.
So simple, in fact, that biologists can't work out whether they can call them living or not.
And they're made of only protein and DNA ...
which means that they're perfect to work out which of the two is the genetic material.
You can imagine a phage like a pipette.
They have a coat like the plastic which sticks into the outside of a cell
and injects the contents of the genetic material into the cell
leaving behind an empty phage or a ghost.
As Miescher had discovered, DNA contains lots of phosphorus which proteins don't have.
So by using radioactive phosphorus, Hershey and Chase were able to tag the DNA and not the protein.
Similarly, protein contains sulfur that DNA doesn't have.
So by using radioactive sulfur, they were able to tag the protein and not the DNA.
They let their radioactive phage particles infect some E. coli cells for a while
before putting the whole mix into a food blender.
Seriously, it was this high tech. And this separated the phage ghosts on the outside of the cell
from the E. coli cells which already had the phage contents inserted into them.
They then separated the lighter phage ghosts from the heavier E. coli cells by centrifuging it.
Or spinning it really fast so that the heavy cells were drawn down to the bottom of the tube.
Then they were able to see what was where.
They found the radioactive sulfur (i.e. the proteins) in the lighter supernatant where all the ghosts were.
Then they found the radioactive phosphorus (i.e. the DNA) in the heavier pellet where all the E. coli cells were.
This means that it was the DNA in the phage that was the genetic material
which matches what Griffith had already found.
But now the biological community believed them.
Now DNA can be celebrated as the miraculous chemical that we all know and love
long before we had even worked out what it looks like.
If you want to find out more about DNA,
I've put some references in the description below.
You can ask questions in the comments below or on Twitter
Or subscribe to keep informed about the latest shed science videos.
Thank you for watching and keep smiling!
*Wops*