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Transcript of Erwin Chargaff
In 1950, Chargaff experimentally determined certain facts that eventually lead to the correct structure of DNA.
In particular, he demonstrated three rules, known as Chargaff's rules, which stated that in DNA:
1. The proportions of adenosine (A) always equals that of thymine (T).
2. The proportions of cytosine (C) always equals that of guanine (G).
3. The number of purines (A+G) is not equal to the number of pyrimidines (C+T)
Chargaff also showed that the ratio of (G+C):(A+T) is held true to any type of organism.
Chargaff's findings strongly suggested that base-pairing existed within DNA between adenine and thymine, and between guanine and cytosine and that other possible pairings such as (A-C, G-T, A-A, T-T, C-C, or G-G) do not occur.
These are the basic facts one has to know to construct an accurate model of the DNA double helix.
By: Harman Rakkar, Chris Haddad, and Stefan Brum
Chargaff was born on August 11th, 1905 in Chernivtsi a city in what was then the southeastern part of the Austro-Hungarian Empire, but that today lies in southwestern Ukraine.
He received a doctorate in chemistry from the Vienna University of Technology (Technische Universität Wien) in 1928.
After graduation he went on to Yale University in the United States, where he investigated the chemical composition of lipids in Mycobacterium tuberculosis, the bacterium causing tuberculosis.
In 1944, Chargaff began his investigations into the composition of DNA. By 1950, he had experimentally determined certain crucial facts that led directly to the correct interpretation of its molecular structure.
Married a woman named Vera Broido in 1928
Had a son named Thomas
Chargaff and his students collected tremendous amounts of DNA samples from different organisms.
Chargaff then used paper chromatography to separate the DNA. The DNA was then placed in an acid which hydrolyzed the phosphodiester bonds and resulted in the backbone breaking up. Once the phosphodiester bonds were broken, the single nucleotides separated and were ready to be analyzed by ultraviolet spectrophotometry.
UV spectrophotometry analyzed exact amounts of bases that were present in the DNA sample.
The results showed that there were not equal amounts of purine bases and pyrimidine bases, which showed that the hypothesis was incorrect in assuming the four bases were present in equal amounts.
This basically meant that the concentration of guanine and cytosine was equivalent to the concentration of adenine and thymine.
Chargaff noticed that even though all bases were not equal amounts, specific bases equaled to each other. Guanine was equivalent to Cytosine and Adenine was equivalent to Thymine. The ratio for adenine + thymine and guanine + cytosine was persistent for all organisms and for each strand of DNA that was separated.
This proportion between a purine base to a pyrimidine base and the fact that was it true for each strand of DNA that Chargaff was not able to visualize, was crucial when Watson and Crick determined the helical structure of DNA
Understanding of DNA today
Chargaff's experiment helps us understand the structure of DNA today.
His experiment allows us to understand:
The base pairs and their complementary pairs (A with T and C with G) found in the DNA when constructing a model, explaining it, or when discussing DNA replication.
It further makes us understand the purines (A and G) and the pyrimidines (C and T).
Chargaff's realization that A = T and C = G, combined with some important X-ray crystallography work by English researchers Rosalind Franklin and Maurice Wilkins, contributed to Watson and Crick's derivation of the three-dimensional, double-helical model for the structure of DNA, and therefore, our visualization of how the helical DNA looks like.
1. When was Erwin Chargaff born? When did he die?
2. What are the complementary nitrogen bases? Purines? Pyrimidines?
3. What are Chargaff's three rules?
4. What are the percentages of A, T, C and G in humans?
BONUS: Given the molecular structures, analyze the bases for 30 seconds. You have one minute to draw Adenine and Thymine. GO!
Erwin died on June 20, 2002 at the age of 96