DNA (deoxyribonucleic acid) is a double-helical polymer composed of two antiparallel polynucleotide strands. Each nucleotide consists of a phosphate group, a 5-carbon sugar (deoxyribose), and a nitrogenous base (adenine, thymine, guanine, or cytosine), forming a sugar-phosphate backbone with bases pairing internally (A-T, G-C).