The structure of DNA molecule described above as visualized by crick and Watson assumes that the helical coiling of the molecule is right handed and this form of DNA has been called the B-form.
Subsequent researches however have indicated that other types of coiling are also possible for the DNA helex.
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These alternative forms of DNA differ from one another in the following respects:
1. Number of residues per turn (n) and
2. Spacing of residues along the helix (£)
The existence of DNA as double helix has also been confirmed by experiments used for measuring the number of base pairs per turn of the helix and this works out to be 10.4 instead of 10 in the B-DNA. This leads to the modification of angle of rotation from 36 to 34.6.
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Thus it is possible that there could be structural variants of DNA with varying values for n and h. Three structural variants of DNA A, B and C are known since a long time and transitions between them are also possible. The following table gives a brief summary of the characters of these forms of DNA.
The forms of DNA are assumed to be found in all molecules depending on the surrounding conditions. In addition to the above D and E forms of DNA are also found very rarely with only 8 and 7.5 base pair per turn. These are found only in some molecules of DNA lacking in Guanine. In contrast A, B and C forms are found in all molecules irrespective of DNA sequence.