Chargaff Rule - Chargaff rule states that the number of purines is always equal to the number of pyrimidines in a given DNA. The relationship is that the number of adenine residue equals the number of thymine, and the number of guanines equals the number of cytosine that is A = T and G = C. Thus we can say A + G = C + T.

X-ray diffraction studies X-ray diffraction studies by M. Wilkins and R. Franklin suggested that DNA could be a helix with two regular periodicities of 3.4 A and 34 A along the axis of the molecules.

3. The double helix has two different grooves. The helix is not absolutely regular. A major (-22A) and a minor (-12A) groove can be distinguished. This feature is important in the interaction between the double helix and the proteins involved in DNA replication and in expression.

4. The nitrogenous bases are stacked towards the inside of the helix. The experimental evidence also indicated that the sugar-phosphate backbone of the molecules is on the outside, with the bases inside the helix.

5. Bases of the two polynucleotides interact by hydrogen  bonding. This is the explanation of Char gaffs base pairs.An adenine residue in one of the polynucleotides is always adjacent to a thymine in the other strand; similarly guanine is always adjacent to cytosine. These two pairs of  bases, and no other combinations are able to form hydrogen bonds between each other. These hydrogen bonds are the only attractive forces between the two polynucleotides of the double helix and serve to hold the structure together.