Chances are you've seen an illustration of DNA's double-helix structure and even pictures of the chromosomes that make up the human genome. But where and how does the famous double helix fit into chromosomes, and how do chromosomes relate to the human body? Here, travel into the tiny world of DNA, all the way down to the level of the atoms that make up a single DNA base.

The human body contains about 100 trillion cells, each working together in a complex symphony of interactions. With the exception of red blood cells, which contain no nucleus and no nuclear DNA, every one of these cells contains the human genome -- a string of three billion A's, C's, G's, and T's. And in every one of the 100 trillion cells, the sequence of these four letters, or bases, is nearly identical.

Although the DNA code from cell to cell is the same, there are many different types of cells within the body, each with a specific function. For example, the long, narrow muscle cell is designed to contract, the branching neuron is designed to send and receive electrochemical impulses, and the squarish cell that lines the wall of the small intestine is designed to filter nutrients from food.

These cells and others in the body are exact copies of their parent cells -- they formed when their parent cells divided. But sometimes cells need to differentiate, or become specialized. Within the first month of embryonic development, cells are changing into different forms. If they didn't, all of the body's cells would be exactly like the single egg cell from which they all originated. This production of new types of cells is the result of DNA "turning on" and "turning off" different sections of the information it stores.

Within every cell (except red blood cells) is a nucleus -- a sphere-like structure separated from the rest of the cell by a membrane. The nucleus acts as the cell's control center, regulating its growth, metabolism, and reproduction. At the heart of this control center is the human genome.

The human genome is comprised of two sets of 23 chromosomes -- 46 chromosomes in all. Each parent contributes a set. About 97 percent of the genome consists of sequences that don't code for proteins and have no known function. Within the rest of the genome are an estimated 70,000 genes.

The single chromosome displayed here and those on the previous screen are shown in their most compacted state -- they're about to divide, along with the cell, through the process of mitosis. When we see pictures of chromosomes, this is usually what we see. The reason is that chromosomes are most visible during this time.

When stained, chromosomes show bands of light and dark areas. The dark bands indicate areas where the structure of the chromosome is dense. Each of the 23 chromosome types has a unique banding pattern. (A chromosome pair has identical banding.) In fact, scientists can identify a chromosome based solely on its banding pattern.

Genes determine whether you have brown eyes or blue, long toes or short, and much, much more. Genes also control everything from how your cells grow to how they interact with one another. A single gene can range in length from as few as 100 DNA bases to as many as several million.

There's a lot of DNA within the nucleus -- about six feet if you could unravel it and stretch it out end to end. To fit such a long molecule within the tiny space of the nucleus, DNA bends and loops in several ways. The largest of these loops results from the helical coiling of chromatin (the thick line in this illustration). This coiling causes the chromosome to resemble a spring.

Chromatin refers to proteins that help organize the long DNA molecule. The protein shown here supports and organizes small loops of DNA.

We've now zoomed in enough to see portions of the DNA strand. The DNA is wrapped around histones -- protein structures that are sometimes depicted as discs. Histones carry a slight positive charge, and DNA carries a slight negative charge. Since opposite charges attract, the DNA is pulled in toward the histones. A nucleosome is a segment of the DNA wrapped around a core of histones.

Here is a view of the double helix -- the subject of Rosalind Franklin's Photo 51. With the help of her photograph, James Watson and Francis Crick were able to piece together the first accurate model of DNA. Shown here is the structure of naked DNA -- DNA without all of the proteins that organize it into chromatin. Note how its structure resembles a twisted ladder. Note also that DNA with a "left-handed" twist, as this has, is a special kind of DNA known as Z-DNA.

The DNA molecule is made up of four bases -- adenine (A), cytosine (C), guanine (G), and thymine (T). Each rung of the DNA ladder consists of two bases. In the DNA molecule, A always pairs up with T, and C always pairs up with G.

The sides of the DNA ladder consist of a long string of sugar and phosphate molecules, to which the bases are attached. Each sugar-phosphate-base combination is called a nucleotide.

A nucleotide is made up of 30 atoms, plus or minus a few, depending on the base. It's no wonder that determining the sequence of bases in the human genome -- all three billion of them -- was such a monumental accomplishment. And though the task of determining the sequence is over, that of understanding the sequence is just beginning. Figuring out how these three billion bases code for a human being will keep researchers busy for many decades to come.