Components of DNA

DNA is a polymer. The monomer units of DNA are nucleotides, and the polymer is known as a "polynucleotide." Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group. There are four different types of nucleotides found in DNA, differing only in the nitrogenous base. The four nucleotides are given one letter abbreviations as shorthand for the four bases. • A is for adenine • G is for guanine • C is for cytosine • T is for thymine DNA Backbone The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3'-hydroxyl and 5'-hydroxyl groups to phosphate groups in ester links, also known as "phosphodiester" bonds. DNA Double Helix DNA is a normally double stranded macromolecule. Two polynucleotide chains, held together by weak thermodynamic forces, form a DNA molecule. Features of the DNA Double Helix • Two DNA strands form a helical spiral, winding around a helix axis in a right-handed spiral • The two polynucleotide chains run in opposite directions • The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a sprial staircase • The bases of the individual nucleotides are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.