Watson and Crick found that the best model that satisfied all the X-ray data was a double helix with the sugar phosphate chain on the outside and the bases on the inside.
The two chains run in an anti parallel fashion with one chain having a 51$$\rightarrow$$ 31 orientation and the other having a 31$$\rightarrow$$ 51 orientation.
The width of the helix was found to be 2 nm. The purine and pyrimidine bases were stacked 0.34 nm apart in a ladder.
The helix made one full turn every 3.4 nm and, therefore, there should be 10 layers of bases stacked in one turn. Since the width of the helix is 2 nm it can accommodate only 2 strands. Each step would contribute a pair of bases, with each base attached to one of the sugar - phosphate backbone.
In a given DNA adenine is equal to thymine and guanine to cytosine. The two strands of DNA are held together by hydrogen bonds.
There are two hydrogen bonds for A = T pairing and three bonds for Cº G pairing. Cº G pairing is more stronger than A = T pairing.
The most crucial outcome of the model was the implicit suggestions for a mechanism by which the genetic material can be copied and transmitted to progeny. A DNA molecule is composed of two strands of nucleotides bound together by hydrogen bonds.
Helical structure is right handed. The fifth (5 - prime, of 5') carbon of the pentose ring is connected to the third (3 - prime, of 3') carbon of the next pentose ring via a phosphate group, and the nitrogenous bases stick out from this sugar-phosphate back bone. By convention, DNA sequences are read from 5' → 3' with respect to the polarity of the strand.
Watson and Crick, model suggested a copying mechanism for DNA. Since one strand is the complement of the other, the two strands can be separated, and each can serve as the template for building a new strand. Watson and Crick were aware of this potential and they wrote in the Journal Nature, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material".
Plans of each organisms are written by using only these four different bases, i.e. by only four kinds of letters. Hence, the plan becomes a long sentence, e.g., the length of DNA in each cell of humans is two meters, which corresponds to 60 trillion letters when calculated in terms of the number of letters. Such a great amount of information is included in a nucleus with a diameter of only 1/100mm.
The number of cells in a man is about 60 trillion and the total length of DNA in a man is 60 trillion x 2m=1.2 x 1011km, corresponding to 5 times the distance between the Sun and Pluto. Thus an organism included both a microscopic world and a universal level of length.
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