All the instructions needed to direct the activities of cells are contained within Deoxyribonucleic acid (DNA), which is a winding double helix structure resembling a twisted ladder. DNA is made of chemical building blocks called nucleotides. Nucleotides consist of 1) a sugar molecule (a deoxyribose), 2) a phosphate group, and one of four nucleotide bases (A, T, G, and C). The double helix of DNA has two linked polynucleotide strands (the sides of the ladder), which are complements of one another. How the nucleotide bases are arranged side by side results in the DNA sequence along each strand (e.g., AATTCCGGA). Adenine (A) and thymine (T) always link together, as do guanine (G) and cytosine (C). This leads to four possible complementary pairings: AT, TA, GC, and CG. Each “rung” of the ladders is made up of two nitrogen bases, pared together by nitrogen bonds, holding the double helix together (Figure 11-1). Each cell in the human body contains about 6 feet of tightly coiled DNA.