The B-form is found at a very high (92%) relative humidity and low ionic strength. Similarly, A-form is found at 75% relative humidity in the presence of N+
, or Cs+
ions and C-form is found at 66% relative humidity in the presence of Li+
)ions. These three forms are assumed to be found in all DNAs.
There are still other forms called D-form and E-form found rarely as extreme variants with only 8 and 7 ½base pairs per turn respectively. These are found only in some DNA molecules lacking guanine. In contrast A, B, and C are found in all DNA molecules irrespective of DNA sequence. As shown in Table 25.1, B DNA is not the only right handed DNA. Similarly, it has been shown that B DNA is not always right handed. B DNA can also be left handed (see RL model later).