DNA, the genetic material, is a macromolecule made of monomers called nucleotides. A DNA molecule resembles a twisted ladder. A sugar-phosphate backbone forms the sides of the ladder. The phosphate group of one nucleotide is covalently bonded to a hydoxyl group on the sugar of the next. Hydrogen bonding between bases creates the rungs of the ladder. Each strand has a free phosphate group at the 5' end and a free hydroxyl group at the 3' end. The numbers refer to the carbons in the sugar part of each nucleotide.