by Alicia M Prater PhD

Created on: February 19, 2009   Last Updated: March 27, 2010

Deoxyribonucleic acid (DNA), less commonly referred to as deoxyribose nucleic acid, was discovered in the 1800s by a Swiss biochemist named Miescher. In 1953, James Watson, Francis Crick, and Rosalind Franklin used breakthrough imaging techniques, specifically X-ray crystallography, to determine that DNA is double-stranded and, famously, helical in structure. Now, almost 60 years later, we understand the macromolecule's structure, even though its secrets are far from being known.

The Chemical Structure of DNA

As the name implies, DNA is a nucleic acid, a macromolecule made of monomeric nucleotides (nitrogen bases) in a chain with a sugar phosphate backbone. The sugar in DNA is deoxyribose. The deoxyribose rings and phosphoric acid of the backbone are connected by covalent bonds and alternate. DNA consists of two chains, or strands, bound in parallel to one another. Each strand is oriented opposite to the other according to the location of the oxygen and hydroxyl groups on phosphoric acid, called the five prime (5') and three prime (3') ends. The nitrogen bases bind to the sugar, in this case deoxyribose, and are known as nucleotides in this configuration.

The Physical Structure of DNA

The size of DNA is on the nanometer scale (approximately 2.2-2.6 nm wide), but it is usually measured in Angstroms (approximately 22 Angstroms wide). The nucleotides project into the center of the molecule, binding to complementary bases on the other strand via hydrogen bonds, and giving the molecule its ladder-like physical appearance. There are four DNA bases, called nucleotides when they are attached to deoxyribose: adenine (A), guanine (G), cytosine (C), and thymine (T). These four include two purines (C and T) and two pyrimidines (A and G). Adenine and thymine bind to one another, and cytosine and guanine bind to one another. Knowing the sequence of one strand automatically reveals the sequence of the other strand due to complementarity.

The physical helical structure of DNA is formed because of the chemical properties of the two parallel, though opposite and complementary, DNA strands as they interact. The DNA molecules present within a cell vary in length and quantity depending on the species it represents.The strands are replicated (copied) and passed on during cell division to serve as the instructions for cellular functions.

The structure of DNA has been described as a twisting ladder, with each rung being made of a nucleotide pair. This structure leaves grooves where

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