DNA has a double helix structure composed of two long strands of nucleotides twisted around each other. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G)
. The two strands run in opposite directions (antiparallel) and are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G)
. The sugar and phosphate groups form the backbone of each strand, linked by phosphodiester bonds between the 3′ carbon of one sugar and the 5′ carbon of the next, giving each strand directionality (5′ to 3′)
. The double helix resembles a twisted ladder, with the sugar-phosphate backbones as the sides and the paired bases as the rungs. The helical structure has a pitch of about 3.4 nanometers and a radius of roughly 1 nanometer
. This structure is stabilized by hydrogen bonding between bases and base- stacking interactions among the aromatic bases
. In summary, DNA’s structure is a double-stranded, antiparallel helix with complementary base pairing, a sugar-phosphate backbone, and a stable helical shape that encodes genetic information
