The replicating molecule of most of the living organisms including humans is DNA (deoxyribonucleic acid).
DNA is the double helical structure made up of nucleotides.
A nucleotide is formed when a deoxyribose sugar is attached to the nitrogenous base (adenine, guanine, cytosine or thymine) with the help of glycosidic bond and to phosphate group with help of ester bond.
Two nucleotides are joined with the help of phospho-di-ester bond in which sugar and phosphate group act as the backbone of the structure and nitrogenous base is used for base pairing.
The two strands of DNA are held together with the help of base pairing. Nitrogen base of one strand makes a hydrogen bond with the nitrogenous base of the second strand.
Adenine always bonds with thymine with the help of 2 hydrogen bonds and guanine always bonds with cytosine with the help of three hydrogen bonds.
The two strands are always anti-parallel to each other.
The average length of one helical turn is around 34 angstroms.