Definition : There are five – different types of DNA exist in nature, which differs from each other with a difference in the normal state, molecular difference etc.
Types of DNA :
(a) A – DNA Found at relative humidity 75%. In high salt concentration or ions concentrations like Na+, K+, Cs+ or in a dehydrated state DNA exists in a form which has 11 nucleotide pairs par turn with a vertical rise of 2.56A0 per base pair.
Widest helical diameter amongst all DNA forms – 23A0 DNA typically right-handed helix with 32.70 rotation per base pair.
(b) B – DNA – Found at 9.25 relative humidity and low ionic strength/low salt concentration. 10 base pairs per turn (340A) titled from the axis of the helix (4-6.300). The distance between two base pairs is 3.4A0 with helical diameter 20A0. Watson –crick’s double helix described as B-form of DNA.
(c ) C – DNA – Found at a relative humidity of 66% and in presence of some ions like Li+ (Lithium). Nearly, 9.33 base pairs per turn. Right-handed helix with 3.320 vertical rises per base pair Diameter of helix is 19A0
(d) D-DNA – Found rarely as an extreme variant. 8 base pairs, negatively titled from the axis of the helix ( Y = -16.70). Axial rise (h) is 3.03A0.
(e) Z – DNA – Occur at a very high concentration of salts. It is a Left – handed helix unlike A, B, and C type DNA. Sugar – Phosphate backbone is zig-zag and the repeating monomer is dinucleotide instead of mononucleotide as in other forms.
There are 12 b.p per turn. Helix diameter is lowest amongst all 18A0. Rotation per base pair is 300
Tabulated: Diff. types of DNA
Diagram – Lf – handed Z – DNA & B – DNA
Note : Z – DNA has very narrow and deep minor grooves and flat major groves. One complete helix is 45A0 unlike 34A0 in B type.
Z – type is the most elongated form of DNA and it is convertible to B type and vice – versa.