The Deoxyribose Nucleic Acid (DNA) molecule is one of the most complex chemical constructs in the universe, and yet its structure is relatively simple to understand. The basic form of the DNA molecule is a double helix, often compared in configuration to a spiral staircase consisting of hundreds of millions of stairs. Each stair on the double helix is called a nucleotide.
A single nucleotide consists of complementary nitrogen base pairs (amines). The bases or amines found in DNA nucleotides come in two varieties, purines and pyridimines. There are two types of purines called Adenine (A) and Guanine (G), and two types of pyridimines called Cytosine (C), and Thymine (T). Each of these bases is an organic compound resulting from the synthesis of amino acids. In DNA nucleotides, adenine is always paired with thymine and cytosine with guanine. Each base pair is joined by a metastable hydrogen bond at the center of the nucleotide. The metastable character of the hydrogen bonds between base pairs in the nucleotide, allows them to be easily made or broken when acted upon by enzymes and is the key facet of the DNA molecule's ability to be replicated and transcribed. At their outer extents, each base pair is attached to a chemical lattice structure known as the backbone with a five carbon sugar called deoxyribose.
THE DNA BACKBONE:
The backbone of the DNA molecule, is a lattice structure equivalent to the rails of the spiral stair case, and is made of interlocking molecules of a five carbon sugar (deoxyribose) and phosphate molecules. A phosphate molecule connects to the sugar molecules above and below it with an ionic bond. Ionic bonds are substantially stronger than hydrogen bonds. This allows the two mono strands of DNA, which form a single helix structure, to exist independently during replication of the molecule.
A complete DNA double helix is comprised of hundreds of millions of nucleotides stacked one atop another. Each set of three vertical nucleotides is called a "codon" and represents a three letter code corresponding to one of twenty amino acids. Actually, there are 64 different possible codon permutations, and only 20 amino acids to be represented. So what happens is, that some amino acids are represented by multiple codons.
Proteins or polypeptide chains are chemicals similar to DNA, but structurally much simpler. Like DNA, proteins are made of amino acids, but all twenty amino acids are found in proteins. In living organisms, proteins do just about everything. Some proteins are the building materials of living organisms. Other types of proteins are called enzymes, and go around braking down other proteins or catalyzing chemical reactions. In all, more than 50,000 different proteins have been identified.
There is another substance called RNA which is a kind of hodgepodge of proteins and nucleotides together, and is responsible for orchestrating all of the chemical reactions going on in a cell. The major function of RNA however, is to read the DNA codons and replicate proteins from it; a process called DNA transcription. In the transcription process, messenger RNA actually reads DNA codons and chemically generates corresponding polypeptide chains or proteins. A sequence of codons in the DNA molecule that code for a specific protein are called a gene. Of the more than 3 billion nucleotides in human DNA, only about 1 percent of them actually code for active genes, and there are only 25 to 30 thousand genes which define the anatomical structure an modulate the metabolic functions of the human anatomy.
In eukaryotic life forms a single DNA molecule is called a chromosome. At either end of the DNA double helix or chromosome there are repetitive sequences of nucleotides which don't code for any amino acids and are called telomeres. Each time a chromosome is replicated some of the telomeres are lost. Once a DNA molecule has lost its telomeres it becomes dysfunctional and the cell it is in dies.
Well, that pretty much wraps up a discussion of DNA from its structural perspective. When you start getting into how DNA and RNA make copies of themselves, and how genes are expressed, things get a whole lot more complex. In fact, there is plenty left to be learned in comprehending how it all works, but now you have at least a basic understanding of the structure of the DNA molecule.
James D. Watson, "DNA The Secret of Life," Alfred A. Knopf, New York, 2003