The outer regions of a DNA molecule are composed of alternating sugar and phosphate groups. The sugar is deoxyribose, which is part of the backbone structure that provides stability and support to the DNA strand. These sugar-phosphate backbones run in opposite directions, creating a structure that is antiparallel.
The rungs of the DNA ladder, which connect the two strands, are made up of nitrogenous bases. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). The bases pair specifically – adenine always pairs with thymine, and cytosine always pairs with guanine. This pairing creates the characteristic double helix structure, allowing the DNA molecule to be tightly coiled and compact.
In summary, the outer regions are formed by sugar and phosphate groups, while the rungs comprise pairs of nitrogenous bases that connect the two strands of DNA, maintaining its essential helical shape.