Stuart E. Ravnik, assistant professor of cell biology and biochemistry at the Texas Tech University Health Sciences Center, summarizes the answer to this seemingly simple question:

Image: Nikolaj Blom and Michael Lappe

HEREDITY'S ALPHABET consists of four letters represented by the nucleotide bases adenine (A), guanine (G), thymine (T) and cytosine (C).

The traits of a living thing depend on the complex mixture of interacting components inside it. Proteins do much of the chemical work inside cells, so they largely determine what those traits are. But those proteins owe their existence to the DNA (deoxyribonucleic acid), so that is where we must look for the answer.

The easiest way to understand how DNA is organized is to start with its basic building blocks. DNA consists of four different sugars that interact with each other in specific ways. These four sugars are called nucleotide bases and have the names adenine (A), thymine (T), cytosine (C) and guanine (G). Think of these four bases as letters in an alphabet, the alphabet of life!

If we hook up these nucleotides into a sequence--for example, GATCATCCG--we now have a little piece of DNA, or a very short word. A much longer piece of DNA can therefore be the equivalent of different words connected to make a sentence, or gene, that describes how to build a protein. And a still longer piece of DNA could contain information about when that protein should be made. All the DNA in a cell gives us enough words and sentences to serve as a master description or blueprint for a human (or an animal, a plant, or a microorganism).

Of course, the details are a little more complicated than that! In practice, active stretches of DNA must be copied as a similar message molecule called RNA. The words in the RNA then need to be "read" to produce the proteins, which are themselves stretches of words made up of a different alphabet, the amino acid alphabet. Nobel laureates Linus Pauling, who discerned the structure of proteins, and James Watson and Francis Crick, who later deciphered the helical structure of DNA, helped us to understand this "Central Dogma" of heredity--that the DNA code turns into an RNA message that has the ability to organize 20 amino acids into a complex protein: DNA -> RNA -> Protein.

To understand how this all comes together, consider the trait for blue eyes. DNA for a blue-eyes gene is copied as a blue-eyes RNA message. That message is then translated into the blue protein pigments found in the cells of the eye. For every trait we have--eye color, skin color and so on--there is a gene or group of genes that controls the trait by producing first the message and then the protein. Sperm cells and eggs cells are specialized to carry DNA in such a way that, at fertilization, a new individual with traits from both its mother and father is created.