The substances making up the sides of the DNA ladder are alternating phosphate groups and deoxyribose sugar molecules. Phosphate groups are inorganic molecules made up of one atom of phosphorus chemically bonded to several oxygen atoms. Deoxyribose is a type of sugar featuring five carbon atoms.
A free nucleotide consists of three phosphate groups bonded to deoxyribose, which is then bonded to one of four nucleotide bases: adenine, thymine, guanine and cytosine. When a nucleotide is added to a growing DNA strand, two of the phosphate groups are detached, leaving the single phosphate group bonded to the sugar. Deoxyribose sugar is similar to the sugar in ribonucleic acid, or RNA, but it is missing an oxygen atom.