Molecular basis of Inheritance — BIOLOGY STD 12 Science — Question

(1) The DNA model has double helical polynucleotide chains or strands which are spirally coiled clock-wise around a common central axis. The thickness of the right handed double helix is about 2 nm (20 Å).
(2) The DNA molecule assumes the shape of a spiral staircase because the nitrogenous bases of successive nucleotide monomers of opposite strands face each other and are joined by hydrogen bonds. The alternating sugar and phosphate molecules of the two strands remain towards the outsides like the long side bars or railings of a staircase. On the other hand, H-bonded bases of opposite strands form the horizontal steps (cross bars) at right angles to central axis. In this way, the two strands cannot be separated easily without completely unwinding them. This type of co coiling is called plectonic.