Let’s begin with Chargaff’s rule.
Chargaff’s rule states that
Rule 1: The amount of Adenine ~equals the amount of Thymine
The amount of Guanine ~equals the amount of Cytosine
The amount of purine = the amount of pyramidine
Rule 2: The amount of A+T ≠ amount of G+C. This ratio varies among
different organisms but same in different tissues of the same organism
Chargaff’s Experiment
He developed a paper chromatography technique to separate and quantify nucleotide
bases
Let us
divide his experiment into two parts
Experiment.
1: Determination of base composition of DNA from different species
First he
isolated DNA from different organisms like Human tissues, E.coli, rat etc. separated and quantified nucleotide bases
Experiment
2: Determination of base composition from different sources of same organism
First he
isolated DNA from different tissues of the same organisms like DNA from Human thumus,
spleen, sperm etc and quantified nucleotide
bases
Procedure:
His
procedure consisted of three steps.
- The first was the separation of the DNA mixture into individual components by paper chromatography.
- Next,the separated compounds were converted into mercury salts.
- And finally, the purines and pyrimidines were identified via their ultraviolet absorption spectra.
From the result he derived Chargaff's
Rule
Watch this video for better understanding
Why is Chargaff's Rule Important?
Explained
base-pairing regularities or "complementarity relationships" of DNA
thus providing the most important clue for the Watson and Crick for deducing
the structure of DNA. At the beginning, Watson and Crick proposed “Like with
like” base pairing where purine pairs with purine and pyramidines with
pyramidines; A=A, G=G etc. Complementary
base pairing provided the most stable orientation of base pairs in the DNA
double helix. This orientation (purine to pyramidine base pairing) best matched
with the X-ray crystallographic data. Thus Chargaff’s Rule provided the
solution for solving this puzzle.
DNA copying
mechanism
Watson and
Crick could explain how DNA makes copies of itself using this base complementarity
relationship. One DNA strand serves as a template for the synthesis of other strand.
Disproved
- Tetra nucleotide Hypothesis by Phoebus Levene
- Triple helical structure of DNA proposed by Linus Pauling
Reference: https://www.jbc.org/article/S0021-9258(20)61522-8/pdf