The DNA Molecule

Double Helix

DNA, or deoxyribonucleic acid, has an elegantly geometric structure that allows it to play its crucial role as the chemical carrier of an organism's genes. Each 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 structures called nucleotides, which, in turn, may be further broken down into three parts: deoxyribose (a five carbon sugar), a phosphate group, and a nitrogenous base. There are four different nitrogenous bases that might be present: thymine, cytosine, adenine, and guanine. These four bases are the foundation of the genetic code. Sometimes represented as T, C, A, and G, these chemicals act as the cell's memory, instructing it on how to synthesize enzymes and other proteins. These four nucleotides encode everything an organism needs to live and protects this information with incredible accuracy. In a human being, each cell holds 46 separate DNA molecules, each containing, on the average, about 160 million nucleotide pairs, yet this massive amount of information is stored and replicated almost flawlessly.

DNA's Backbone
The backbone of the long DNA molecule is quite strong. It is made up of alternating sugars and phosphates linked through oxygen atoms. The bonds between these structures are covalent, and therefore difficult to break. This strong backbone helps guard the genetic information against destruction or mutation.

The Nitrogenous Bases
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 (the nitrogenous bases would be the rungs).

Thymine Cytosine Pyrimidines

Adenine Guanine Purines

Thymine and cytosine are known as pyrimidines, while adenine and guanine are purines. Pyrimidine molecules form six cornered rings - purines are a combination of a five cornered ring and a six cornered ring. Purines, then, are the much larger of the two. Because of their sizes, two purines could never bond with each other in a DNA strand because they would be too large and pyrimidines couldn't because they would be too small to reach each other. This ensures that purines only match up with pyrimidines in the DNA structure and vice-versa. This factor, along with the fact that adenine and thymine must form two hydrogen bonds to be stable while guanine and cytosine must form three, makes the base pairing system an extremely simple and dependable one: the A-T and C-G pairs are the only ones physically possible.

The bonds between the nitrogenous bases that hold the two parts of the DNA molecule are relatively weak ones involving hydrogen atoms. This weakness makes it possible to separate the molecule into its two parts for easy access of its information, which is discussed later.