Q.Write the characteristic feature of DNA

Step 1. Antiparallel double helix. DNA consists of two polynucleotide chains that are antiparallel - one strand runs 5'->3' while the other runs 3'->5' - and coil around a common axis to form a right-handed double helix (the predominant B-form).

Step 2. Complementary base pairing. The two strands are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds, following Chargaff's rule (A = T, G = C, so purines always equal pyrimidines in amount).

Step 3. Major and minor grooves. Because the two sugar-phosphate backbones protrude from the base pairs at unequal angles (about 120 degrees and 240 degrees), the helix surface shows a major groove and a minor groove.

Step 4. Helical dimensions. Each base pair is spaced 0.34 nm apart along the axis, and one complete turn of the B-form helix spans about 3.4 nm and contains 10 base pairs; the helix has a diameter of about 20 angstrom. …

List the structural (antiparallel double helix, grooves, dimensions) and chemical (base-pairing, Chargaff's rule, …

• Stating only 'DNA is a double helix' without the antiparallel orientation or the specific base-pairing rule. …