Going to the original experiment of Chargaff....
1. Cells-get DNA
2. Purify DNA
3. Subject it to mild acid treatment-to break the phosphodiester bond.
This released the free nucleotides.
4. Subjected to chromatography
5. Quantified the nucleotides
He repeated this for different organisms...
For e.g.. The real values found by Chargaff with the human cells was
A:T-1.00
G:C-1.00
For E.Coli
A:T-1.99
G:C-0.99
This is essentially what is called as Chargaff's Rule. But to be
precise it should be called as Chargaff's First Parity Rule (there is a
second parity rule of Chargaff too!)
There are actually 4 rules attributed to Chargaff........Of the four
rules 3 are species invariant and 1 was species variant (varied with
species)
The first
Parity Rule: (Species
invariant)
For Duplex DNA they identified a
species-invariant component of the base composition, (1950,1951)
%A
= %T and %C = %G.
The Cluster Rule:
(Species
invariant)
Individual bases are clustered to a greater extent than expected on a
random basis.(1963)
The second Parity Rule:
(Species
invariant)
The second species-invariant observation was that Chargaff's first
parity rule also applies, to a close approximation, to single-stranded
DNA (his "second parity rule"). If the individual strands of a DNA
duplex are isolated and their base compositions determined, then %A = %T and %C =
%G.
GC rule: (Species
variant)
Finally, there was the observation that the ratio of C + G to the total
bases (A+C+G+T) tends to be constant in a particular species, but
varies between species.(1951,1979)
Even though he found the constancy of proportions of the percentage of
A
to the percentage of T; similarly for C and G percentage.....He did not extend his findings to suggest
that Purines is equal to the pyrimidines or that A bonded only to T and
that C bonded only to G. This is clear when Crick discusses Chargaff's rule*,"I wasn't aware of Chargaff's rules when he said them, but the effect on me was quite electric because I realized immediately that if you had this sort of scheme that John Griffiths was proposing, of adenine being paired with thymine, and guanine being paired with cytosine, then you should get Chargaff's rules. I was very excited, but I didn't actually tell Chargaff because it was something I was doing with John Griffiths. There was a sort of musical comedy effect where I forgot what the bases were and I had to go to the library to check, and I went back to John Griffiths to find out which places he said. Low and behold, it turned out that John Griffith's ideas fitted in with Chargaff's rules! This was very exciting, and we thought 'ah ha!' and we realized - I mean what anyone who is familiar with the history of science ought to realize - that when you have one-to-one ratios, it means things go to together. And how on Earth no one pointed out this simple fact in those years, I don't know. "
No where did Chargaff actually mention about the equality of the
purines and pyrimidines. When Watson and Crick proposed their Double
helix structure, they mentioned that the two strands of DNA were joined
by the hydrogen bonds between a purine of one strand and the pyrimidine
of the other strand and this is called as the Watson-Crick Base
pairing, based on Chargaff's First parity Rule.
To summarize till now:
- Chargaff found that different species of
organisms have different DNA nucleotide compositions
- Chargaff’s rule states that the fraction of
nucleotides that makes up an organism’s DNA always behaves the rule:
the fraction of A’s = the fraction of T’s, and the fraction of G’s =
the fraction of C’s.
Adenine bonds with Thymine and guanine
bonds with Cytosine or that the purines is equal to the pyrimidines was
not proposed by Chargaff even though his findings were instrumental in
this deduction. He equated the molar proportional relationship Adenine
and thymine; cytosine and guanine.
Chargaff's First Parity Rule (or simply Chargaff's Rule) states
- Fraction of Adenine = Fraction of Thymine.
- Fraction of cytosine = Fraction of Guanine.
Actually, the
structure of DNA was deduced due to the results of Franklin
Rosalind (did you know that she was a lady and had an uncanny ability
to
get clear X-ray diffraction films of individual chromatin?) and due to Chargaff's first parity rule, apart from many other's contributions.
So conclusively
the answer is
A=T and G=C and A/T = G/C and is approximately (experimentally
found to be approximate because of errors) equal to one.
So the exact choice is 2.
I am bound to add a tribute to Rosalind since inspite of her role which culminated in the
discovery of the structure of DNA, she did not receive the Nobel Prize
in 1963 which was shared between Watson, Crick and Wilkins (who worked
with Rosalind). She was not even mentioned in the Nobel Prize speech.
It was only Wilkins who made a brief gratitude remark about Rosalind's
role. She died before Nobel Prize was announced for the discovery of
Structure of DNA and Sir Alfred Nobel's did not allow sharing of the
prize among more than three people and also did not allow awarding the
prize posthumously! So her works and role are usually not given the
due respect deserving.
Note: *Reference for Francis Crick's speech transcript:
The DNA Story - 1973
http://osulibrary.orst.edu/specialcollections/coll/pauling/dna/videos/dnastory06.html
