| "Two DNA strands intertwine to form a double helix. Each strand has a backbone composed of phosphates and sugars to which the bases are attached. The bases form the core of the double helix, while the sugar/phosphate backbones are on the outside. The two grooves between the backbones are called the major and minor groove based on their sizes. Most proteinDNA contacts are made in the major grove, because the minor groove is too narrow. The DNA backbone is assembled from repeating deoxyribose sugar units that are linked through phosphate groups. Each phosphate carries a negative charge, making the entire DNA backbone highly charged and polar. A cyclic base is attached to each sugar. The bases are planar and extend out perpendicular to the path of the backbone. Pyrimidine bases are composed of one ring and purine bases of two rings. Adjacent bases are aligned so that their planar rings stack on top of one another. Base stacking contributes significantly to the stability of the double helix. In a double helix, each base on one strand is paired to a base on the other strand that lies in the same plane. In these base pairing interactions, guanine always pairs with cytosine, and thymine with adenine. A GC pair is stabilized by three hydrogen bonds formed between amino and carbonyl groups that project from the bases. In contrast, an AT pair is stabilized by two hydrogen bonds. The specificity of base pairing—that is, C always pairing with G, and A always pairing with T—ensures ... |
