You go through a door in the nucleus’ outer skin, or membrane, and look around you. Dominating this chamber are 46 chromosomes. Arranged in identical pairs, they vary in height, but the pair nearest you is about 12 stories tall (1). Each chromosome has a pinched place near the middle, so it looks a bit like a link sausage but is as thick as a massive tree trunk. You see a variety of bands running across the model chromosomes. As you draw closer, you see that each horizontal band is divided by vertical lines. Between those are shorter horizontal lines (2). Are they stacks of books? No; they are the outer edges of loops, packed tightly in columns. You pull at one of them, and it comes free. You are amazed to see that the loop is composed of smaller coils (3), also neatly arranged. Within those coils is the main feature of all of this​—something resembling a long, long rope. What is it?

THE STRUCTURE OF AN AMAZING MOLECULE

Let us simply call this part of the model chromosome a rope. It is about an inch (2.6 cm) thick. It is looped tightly around spools (4), which help to form the coils within coils. These coils are attached to a kind of scaffold that holds them in place. A sign on the display explains that the rope is packed very efficiently. If you were to pull the rope from each of these model chromosomes and lay it all out, from end to end it would stretch about halfway around the earth!a

One science book calls this efficient packaging system “an extraordinary feat of engineering.”¹⁸ Does the suggestion that there was no engineer behind this feat sound credible to you? If this museum had a huge store with millions of items for sale and they were all so tidily arranged that you could easily find any item you needed, would you assume that no one had organized the place? Of course not! But such order would be a simple feat by comparison.

In the museum display, a sign invites you to take a length of this rope in your hands for a closer look (5). As you run it between your fingers, you see that this is no ordinary rope. It is composed of two strands twisted around each other. The strands are connected by tiny bars, evenly spaced. The rope looks like a ladder that has been twisted until it resembles a spiral staircase (6). Then it hits you: You are holding a model of the DNA molecule​—one of the great mysteries of life!

A single DNA molecule, tidily packaged with its spools and scaffold, makes up a chromosome. The rungs of the ladder are known as base pairs (7). What do they do? What is all of this for? A display sign offers a simplified explanation.

THE ULTIMATE INFORMATION STORAGE SYSTEM

The key to the DNA, the sign says, lies in those rungs, the bars connecting the two sides of the ladder. Imagine the ladder split apart. Each side has partial rungs sticking out. They come in only four types. Scientists dub them A, T, G, and C. Scientists were amazed to discover that the order of those letters conveys information in a sort of code.

You may know that Morse code was invented in the 19th century so that people could communicate by telegraph. That code had only two “letters”​—a dot and a dash. Yet, it could be used to spell out countless words or sentences. Well, DNA has a four-letter code. The order in which those letters​—A, T, G, and C—​appear forms “words” called codons. Codons are arranged in “stories” called genes. Each gene contains, on average, 27,000 letters. These genes and the long stretches between them are compiled into chapters of a sort​—the individual chromosomes. It takes 23 chromosomes to form the complete “book”​—the genome, or total of genetic information about an organism.b