The Double Helix:

Every living thing has DNA -- deoxyribonucleic acid -- which comes in the form of a long, threadlike molecule. A strand of human DNA found inside a chromosome is about three feet long -- but so thin that it is measured in angstroms, or hundred-millionths of a centimeter.

Imagine that each of these strands is made up of two helixes -- tight spirals that look almost like tubes -- of molecules. Each helix is made up of alternating phosphate and deoxyribose (sugar) units. Now, picture two of these helixes twisted tightly together to form one strand. This is the famous double helix, the characteristic shape of DNA, discovered in 1958 by James Watson and Francis Crick.

The helixes have links as a ladder's sides are linked with rungs. There are two different phosphates (adenine and guanine) and two sugars (thymine and cytosine), called "bases," that make up all of these rungs, in every plant and animal and microorganism in the world. Adenine (A) and thymine (T) are complementary, as are cytosine (C) and guanine (G). This means that where an A occurs on one helix, immediately opposite it on the other helix must lie a T. Where C occurs, a G occurs opposite. Looking at the illustration above, you can see nine "rungs" -- reading from the top, TA, then CG, GC, TA, TA, AT, TA, AT, CG.

Below that last CG, the "rungs" are starting to come apart. Finally, the bonds are completely broken, and each ribbon starts trying to attract a new set of complementary molecules. This is how DNA replicates -- by "unzipping" and then assembling a new ribbon of sugars and phosphates, so it can become a double helix again, with a new set of partners. The replication process is made possible through enzymes, other molecules, and interrelated metabolic processes. It is necessary because when cells divide and multiply, each cell needs its own DNA.

Even though we have perhaps 40 yards of DNA in every one of our cells, only about 4 yards of that functions in ways that determine our genetic (inherited) traits. The rest is sometimes called "junk DNA" -- how it got there and what it does is one of the many unknowns in science.

The functioning parts of DNA are called genes. . Most genes appear to have 5-10,000 base pairs (AT or CG pairs), although some have several hundred thousand . We have not yet "found" all the genes.

What Does DNA Determine?

How does a long row of AT's and CG's translate into leaves and eyeballs and fur and hummingbird wings? To shorten a stunningly complex process into a single statement, we could say that genes determine traits by "coding for" a sequence of amino acids that, in turn, make up different proteins.

This means that through a chain of chemical reactions, hundreds of amino acids stack up to build proteins. Most proteins are folded into three-dimensional shapes that depend both on their amino acid composition and on their cell environment -- that is, concentrations of different salts, the presence of metal molecules, and other molecules such as sugars and fats, in the cells. How proteins function depends both on their amino acid sequence -- the part controlled by DNA -- and their shape, which is influenced by other factors.

These other factors, in turn, may be influenced by the organism's own behavior and its external environment. This is why there is uncertainty over how much DNA actually determines. Even in the case of diseases that are clearly genetic, for instance, the disease may take quite a different course from one person to the next, because biology is so complex and people's lives are so different.

SOURCE: Exploding the Gene Myth by Ruth Hubbard and Elijah Wald, Beacon, 1993