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The Crick and Watson model for DNA, 1953

Interest in the structure of DNA quickly increased after more and more people thought it could be the genetic material. Francis CrickBiography and James WatsonBiography proposed a model for the structure of DNA that was accepted by everybody. They used the results of a number of different experiments.

X-ray diffraction experiments :

X-rays passing through a crystallised molecule are diffracted and sent in different directions. Analysis of the directions gives important information on the structure of the molecule. This technique has been successfully used for DNA, and is in constant use for proteins. Rosalind FranklinBiography performed the experiments that gave Crick and Watson evidence for their hypothesis.

Chargaff, 1952: base ratio analysis.

The investigation of the organic base composition by Edwin Chargaff, was another breakthrough in the route to the structure of DNA. The organic bases are adenine (A), guanine (G), thymine (T), and cytosine (C). When calculating the percentage of each base for different species, the results were as follows:

Source of DNA Adenine Thymine Cytosine Guanine
Human 30.9 29.4 19.8 19.9
Hen 28.8 29.2 21.5 20.5
Salmon 29.7 29.1 20.4 20.8
Locust 29.3 29.3 20.7 20.5
Wheat 27.3 27.1 22.8 22.7
Yeast 31.3 32.9 17.1 18.7
E. coli 24.7 23.6 25.7 26.0

Chargaff's analysis led him to important conclusions. Before I tell you which, try answering these simple questions :

  1. Calculate the number of purine bases, and compare with the number of pyrimidine bases. What is your conclusion?
  2. Compare the number of thymine bases with the number of adenine bases. What is your conclusion?
  3. Compare the number of guanine bases with the number of cytosine bases. What is your conclusion?
  4. What model would fit this important result best?

Chargaff's conclusions :

1) Calculate the number of purine bases, and compare with the number of pyrimidine bases. What is your conclusion?

In humans , the number of purine bases (A + G) is 50.8. The number of pyrimidine bases (C+ T) is 49.2. If we calculate these numbers for each species, we find that A+G = C+T.

2) Compare the number of thymine bases with the number of adenine bases. What is your conclusion?

A = T

3) Compare the number of guanine bases with the number of cytosine bases : What is your conclusion?

C = G

4) What model would fit this important result best?

These observations are best interpreted as adenine being always paired with thymine and guanine always paired with cytosine.


Crick and Watson model

Crick and Watson built a model of DNA as a double helix comprised of two polynucleotide chains coiled around the same axis and interlocking. The sugar-phosphate chains from the backbone of the helix and are on the outside. The bases pair together on the inside, held by hydrogen bonds between specific pairs of bases. This is shown on the diagram on the right.

The base pairing has the tremendous consequence that the sequence on one chain determines the sequence on the other chain. The two strands are said to be complementary.

The sugar molecules attached to two complementary bases face opposite directions: the two strands are antiparallel.

Non-helical models of DNA have been proposed : the two strands are alongside each other. I have drawn two here.


Conclusion

A quote from the Crick and Watson paper in 'Nature':

"It has not escaped our notice that the specific pairing of bases we have postulated, immediately suggest a possible copying mechanism for the genetic material"

What are they referring to? Each strand of the double helix can act as a template to synthesise the opposite strand.

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