Sometimes one or more nicks or breaks may be present on one or both strands, for example DNA of phage PM2 (Fig. Thus, a closed circle contains two unbroken complementary strands. The two ends of the double helix get covalently sealed to form a closed circle. Circular and Super Helical DNA:Īlmost in all the prokaryotes and a few viruses, the DNA is organised in the form of closed circle. (d) The dsDNA always remains in linear helical form, while the ssDNA remains in circular form however, it becomes double stranded only during replication (i.e. A=T and G=C, in ssDNA the composition of A, T, G, C is in proportion of 1:1.33:0.98:0.75. (c) Base pair composition in dsDNA is equal i.e. (b) The dsDNA resists the action of formaline due to closed reactive site, while the ss DNA does not resist it due to exposed reactive sites. The Z-Form of DNA (Z-DNA) or Left Handed DNA: the base pairs are displaced backwardly with respect to the axis of DNA helix. There is pronounced negative tilt of base pairs by 16.7° as compared to C form i.e. This form is also called poly (dA-dT) and poly (dG-dC) form. Total number of base pairs per turn of helix is eight. The D-form of DNA is found rarely as extreme vanants. The base pairs show pronounced negative tilt by 7.8°. As compared to A-and B-DNA, in C-DNA the number of base pairs per turn is less i.e. The C-form of DNA is found at 66% relative humidity in the presence of lithium (Lit+) ions. The A-form is metastable and quickly turns to the D-form. Due to this displacement the depth of major groove increases and that of minor groove decreases. It contains eleven base pairs as compared to ten base pairs of B-DNA which tilt from the axis of helix by 20.2°. The A-form of DNA is found at 75% relative humidity in the presence of Na+, K+ or Cs+ ions. The B-form of DNA is metabolically stable and undergo changes to A, C or D forms depending on sequence of nucleotides and concentration of excess salts. The base pairs are perpendicular to longitudinal axis of the helix.