DNA Structure

Elements | Molecular Structure | Backbone | Base Pairs

DNA (deoxyribonucleic acid) contains the genetic instructions for the development and function of living things. The DNA segments that carry genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the expression of genetic information. A DNA molecule is made up of two very long polymers connected by the bonding of hydrogen atoms and coiled in the shape of a double helix. Each of the two polymers contains many repeating structures called nucleotides, which may be further broken down into three parts: deoxyribose (a five carbon sugar), a phosphate group, and a nitrogenous base.

Four different nucleotides exist that usually only differ in their nitrogenous bases. The four bases, thymine (T), cytosine (C), adenine (A), and guanine (G) comprise the foundation of the genetic code. These chemicals act as the cell's memory, instructing the cell on how to synthesize enzymes and other proteins. While protecting this information with incredible accuracy, the four nucleotides encode everything an organism needs to live by. In a human being, each cell holds 46 separate DNA molecules, each containing about 160 million nucleotide pairs.

The backbone of the long DNA molecule is quite strong, which guards its genetic information from destruction or mutation. It is made up of alternating sugars and phosphates linked through oxygen atoms. The deoxyribose sugars are joined at both the 3'-hydroxyl and 5'-hydroxyl groups to phosphate groups in ester links, also known as asymmetric "phosphodiester" bonds, providing each strand of DNA with a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand, or in other words, they are antiparallel to one another. The asymmetric ends of a strand of DNA bases are referred to as the 5' (five prime) and 3' (three prime) ends.

The nitrogenous bases connect to the backbone by bonding to the sugars. As they stick out from the long backbone they attract a complimentary base (adenine bonds with thymine, cytosine to guanine), and thus it is through the weak hydrogen bonding of these bases that the two long polymers of the DNA molecule are connected. Because of the nature and shape of these connections, DNA spirals into a double helix, a shape that can be best described as a twisted ladder.

As the DNA strands wind around each other in a right-handed spiral, they leave gaps between each set of phosphate backbones, revealing the sides of the bases inside. There are two of these grooves, the major and the minor groove, twisting around the surface of the double helix. The narrowness of the minor groove means that the edges of the bases are more accessible in the major groove. As a result, proteins like transcription factors that can bind to specific sequences in double-stranded DNA usually read the sequence by making contacts to the sides of the bases exposed in the major groove.