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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 discovered all of the components of DNA. Like for example: adenine, guanine, thymine, cytosine, deoxyribose phosphate defined phospgate-sugar-base units called nucleotides. -
Levene's Tetranucleotide
Levene proposed that there were four nucleotides per molecule
Said DNA could not store the genetic code because it was chemically far too simple.
But this theory was incorrect.
Levene died in 1940. -
Frederick Griffith and his Transformation Experiment
Studied the epidemiology and pathology of 2 strains of Streptococcus pneumoniae .
In January 1928 reported the first widely accepted demonstrations of bacterial transformation
Griffith used two strains of Streptococcus:
Type S: virulent (deadly)
Type R: non-virulent (harmless)
Observed bacterial transformation but did not understand the mechanism. -
Cause of Transformation
Avery, MacLeod and McCarty, 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:
a protease (destroys protein)
a DNAse (destroys DNA). -
Double helix
In 1951, Watson and Crick wrote a paper in which they described DNA as a double helix with sugars and phosphates at the center and the nucleobases facing the outside
This model was quickly shown to be incorrect and in fact it made no chemical sense. -
Hershey-Chase Experiments
Used phages and radiolabeled phosphorus and sulfur
Hershey and Chase concluded that DNA, not protein, was the 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. -
Triple Helix
Linus Pauling and Robert Corey proposed a triple helix structure for DNA.
With the phosphates and the sugar inside and the nucleobases on the outside.
This theory turned out to be incorrect. -
Counting Nucleobases
Erwin Chargoff was Counting Nucleobasesm in 1952.
Used paper chromatography and UV spectroscopy to examine the abundance of the nucleobases and he started to notice something very odd...
Came to be known as "Chargoff's Rules"
Amounts of Adenine = Amounts of Thymine
Amounts of Cytosine = Amounts of Guanine
Always in every species. -
So it's the DNA
The problem was on to determine the structure of DNA in cells and determine how it codes for proteins and how it replicates
DNA exists in two forms:
A form (dry form)
B form (wet form, as DNA exists in cells). -
Photo 51
Rosalind Franklin took a lot of photographs of the B form of the DNA. She figured out how to see the wet form, the form that exists in cells. The photo, 51, shows very clearly the x in the middle that is the sign of a double helix. -
Actual model
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.
