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Discovery of Nucleic Acids
Friedrich Miescher isolated the genetic material from white blood cell nuclei. He noted it had an acidic nature and called it nuclein. -
Discovery of DNA components
Phoebus Levene determined the components of DNA and defined phosphate-sugar-base units called nucleotides. -
Levene's Tetranucleotide
Levene proposed that there were four nucleotides per molecule. DNA could not store the genetic code because it was chemically far too simple. -
Griffith's Transfomation Experiment
Frederick Griffith reported the first widely accepted demonstrations of bacterial transformations. He used two strains of Streptococcus:
- Type S: virulent (deadly)
- Type R: non-virulent (harmless) In his experiment:
- Rought strain doesn't kill the mouse.
- Smooth strain kill the mouse.
- Heat killed smooth strain doesn't kill the mouse.
- Rought strain & heat-killed smooth strain kill the mouse. The question was: How did that happen? -
Answering Griffith's Question
3 people answered the question: Oswald Avery, Colin MacLeod and Maclyn McCarty. They determined the cause of the transformation in Griffith's Experiment.
They took live R and heat-treated S and mixed it with one of two enzymes:
- Proteases which destroys protein.
- DNase which destroys DNA. -
Double Helix?
Watson and Crick wrote a paper in which they described DNA as a double helix with sugars and phosphates at the centre and the nucleobases facing the outside.
This model was quickly shown to be incorrect and in fact it made no chemical sense. -
Counting Nucleobases
Erwin Chargaff used paper chromatography and UV spectroscopy to examine the abundance of the nucleobases and he started to notice something very odd... (see the image) -
Chargaff's Rules
The Chargaff's rules said that the amount of adenine and thymine were always in balance, and the amount of cytosine and guanine, always in balance. -
Hershey and Chase Experiments
Alfred Hershey and Martha Chase, had some experiments going on with phages. Bacteriophages are viruses that infect bacteria.
They use bacteria cell (host) and then they took (bacterio)phage with radioactive protein capsule, or they used radioactive DNA to follow the movement of DNA during the infection. (See the image to know how the experiment was) -
Conclusions of Hershey and Chase
- DNA, not protein, was genetic material.
- A protective protein coat was formed around the bacteriophage, but the internal DNA is what conferred its ability to produce progeny inside bacteria.
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Photo 51
X-ray diffraction image of DNA Photo 51 by Rosalind Frankling.
But somehow, Maurice Wilkins got Photo 51 from her desk in Kings College and managed to get it to Watson and Crick in Cambridge. -
Triple Helix?
Linus Pauling and Robert Coley proposed a triple helix structure of DNA (see the image).
That turned to be incorrect. -
A Structure for Deoxyribose Nucleic Acid
With Photo 51 Watson and Crick built the model in 3D based in the image. And they also describe what DNA looks like in a paper. -
3 Papers Describing the Structure of DNA
3 papers were published describing the real structure of DNA.
- The first one by Watson and Crick.
- The second one by Stokes and Wilkins.
- The third one by Rosalind Franklin and her assistant Raymond Gosling. -
Nobel Prize
Watson, Crick and Wilkins got the Nobel Prize in Physiology or Medicine for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material.
Rosalind Franklin did not share in the Nobel Prize is because she had already passed away, and Nobel Prizes are only given to living people. Almost more disappointing is that neither of the three acknowledged the importance of her work in their discovery. -
Nowadays we know that...
- DNA is a double-stranded helix
- 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.