7.3: DNA—The Secrets in Its Structure

DNA (deoxyribonucleic acid) is a chain of chemical units, each made of a sugar, a phosphate (a combination of phosphorus and oxygen), and a base (a nitrogen-bearing chemical which looks like a second cousin to an amino acid):

These units join with one another to form a chain, like this:

The chain joins side-to-side with another chain. Their bases, which look like “arms” protruding outward, link together like this:

The result is a kind of chemical ladder. The sugar-phosphate parts of the units are now the sides of the ladder. The bases form the rungs. And now the ladder twists into a spiral, thus:

This spiral—the double helix called DNA—is the master control of life. Human DNA is estimated to have some 3 billion rungs in its chemical ladder. Within this ladder, lie about 20,000 sets of instructions to make an astonishingly wide array of human proteins.

Where do the instructions lie in DNA? As scientists searched for the answer, it was noted that the bases that make up the rungs of the DNA ladder are varied—that actually there are four kinds of bases. The sequence of these bases seemed random, but the scientists found that the order was the coded plan of life.

So let’s focus for a moment on this tiny part of DNA. To give us a simple way of seeing how the code was discovered, let’s look at the four kinds of bases as the four suits of playing cards—clubs, diamonds, hearts, and spades:

One fact which struck the scientists was that although the bases varied, when they linked up as the “rungs of the ladder,” they paired consistently, always in the same pattern. One kind of base will join only with one other kind of base. In terms of our playing-card labeling of the four kinds of bases, clubs link only with spades, and hearts link only with diamonds.

But this wasn’t the complete answer—the consistent pattern of the base-matching alone couldn’t provide enough variables to code for all 20 amino acids. There had to be more to the code. Scientists were stumped, but not for long.

The breakthrough was the discovery that the code didn’t lie in reading one base at a time, but rather three bases at a time. Each of these groupings of three is called a triplet, and the triplets changed the mathematics drastically, providing 64 possible combinations of the four bases, such as this one:

Now the answer to the puzzle rapidly emerged. Sixty-four triplets are more than enough to code for 20 amino acids. By this code—the genetic code—the DNA in each cell can use its sequence of bases to call for the amino acids in a specific order and number. Such a chain of amino acids, as we’ve seen, is a protein.

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The sequence of bases in DNA are read 3 at a time; the set of 3 (a triplet) codes for an amino acid.

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