DNA is genetic material in most of organisms (exception: RNA virus). It is a double-helix and has two strands running in opposite directions. Each chain is a polymer of subunits called nucleotides (polynucleotide).
A DNA strand has a backbone made up of (deoxy-ribose) sugar molecules linked together by phosphate groups. The 3′ C of a sugar molecule is connected through a phosphate group to the 5′ C of the next sugar. This linkage is also called 3′-5′ phosphodiester linkage. All DNA strands are read from the 5′ to the 3′ end where the 5′ end terminates in a phosphate group and the 3′ end terminates in a sugar molecule.
Each sugar molecule is covalently linked to one of 4 possible bases (Adenine, Guanine, Cytosine and Thymine). A and G are double-ringed larger molecules (called purines); C and T are single-ringed smaller molecules (called pyrimidines).
In the double-stranded DNA, the two strands run in opposite directions and the bases pair up such that A always pairs with T and G always pairs with C. The A-T base-pair has 2 hydrogen bonds and the G-C base-pair has 3 hydrogen bonds.